FAILED: patch "[PATCH] slub: improve bit diffusion for freelist ptr obfuscation" failed to apply to 4.19-stable tree

PATCH [email protected] 3 participants 5 messages Apr 10, 2020 – Apr 14, 2020

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Stable branch Memory management Slab allocator

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[email protected]

author maintainer

2020-04-10 08:09 UTC #0 lore raw

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation

Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak in that the ptr and ptr address were usually so close that the first XOR would result in an almost entirely 0-byte value[1], leaving most of the "secret" number ultimately being stored after the third XOR. A single blind memory content exposure of the freelist was generally sufficient to learn the secret.

Add a swab() call to mix bits a little more. This is a cheap way (1 cycle) to make attacks need more than a single exposure to learn the secret (or to know _where_ the exposure is in memory).

kmalloc-32 freelist walk, before:

ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...

after:

ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)

[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html

Fixes 2482ddec670f ("mm: add SLUB free list pointer obfuscation")

Reported-by Silvio Cesare <[email protected]>

Signed-off-by Kees Cook <[email protected]>

Signed-off-by Andrew Morton <[email protected]>

Cc Christoph Lameter <[email protected]>

Cc Pekka Enberg <[email protected]>

Cc David Rientjes <[email protected]>

Cc Joonsoo Kim <[email protected]>

Cc <[email protected]>

Link http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook open here

Signed-off-by Linus Torvalds <[email protected]>

mm/slub.c +1 -1

index fc911c222b11..bc949e3428c9 100644
@@ -259,7 +259,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
 	 * freepointer to be restored incorrectly.
 	 */
 	return (void *)((unsigned long)ptr ^ s->random ^
-			(unsigned long)kasan_reset_tag((void *)ptr_addr));
+			swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
 #else
 	return ptr;
 #endif

Kees Cook

2020-04-13 17:00 UTC #1 lore raw

Hi!

I've backported this to v4.19 (and v4.14). Those backports require that d5767057c9a76a29f073dad66b7fa12a90e8c748 is cherry-picked into stable for v4.19 and v4.14 as well. I will send patches...

-Kees

Quoted 75 lines · On Fri, Apr 10, 2020 at 10:09:02AM +0200, [email protected] wrote:

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation
Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak in that the ptr and ptr address were usually so close that the first XOR would result in an almost entirely 0-byte value[1], leaving most of the "secret" number ultimately being stored after the third XOR. A single blind memory content exposure of the freelist was generally sufficient to learn the secret.
Add a swab() call to mix bits a little more. This is a cheap way (1 cycle) to make attacks need more than a single exposure to learn the secret (or to know _where_ the exposure is in memory).
kmalloc-32 freelist walk, before:
ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...
after:
ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)
[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html
Fixes 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by Silvio Cesare <[email protected]>
Signed-off-by Kees Cook <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Cc Christoph Lameter <[email protected]>
Cc Pekka Enberg <[email protected]>
Cc David Rientjes <[email protected]>
Cc Joonsoo Kim <[email protected]>
Cc <[email protected]>
Link http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook open here
Signed-off-by Linus Torvalds <[email protected]>
mm/slub.c +1 -1
index fc911c222b11..bc949e3428c9 100644
@@ -259,7 +259,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
 	 * freepointer to be restored incorrectly.
 	 */
 	return (void *)((unsigned long)ptr ^ s->random ^
-			(unsigned long)kasan_reset_tag((void *)ptr_addr));
+			swab((unsigned long)kasan_reset_tag((void *)ptr_addr)));
 #else
 	return ptr;
 #endif

Signature · 1 line

Kees Cook

Sasha Levin

maintainer

2020-04-14 02:40 UTC #2 lore raw

In reply to [email protected]

Quoted 49 lines · On Fri, Apr 10, 2020 at 10:09:02AM +0200, [email protected] wrote:

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation
Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak in that the ptr and ptr address were usually so close that the first XOR would result in an almost entirely 0-byte value[1], leaving most of the "secret" number ultimately being stored after the third XOR. A single blind memory content exposure of the freelist was generally sufficient to learn the secret.
Add a swab() call to mix bits a little more. This is a cheap way (1 cycle) to make attacks need more than a single exposure to learn the secret (or to know _where_ the exposure is in memory).
kmalloc-32 freelist walk, before:
ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...
after:
ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)
[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html

Quoted 11 lines

Fixes 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by Silvio Cesare <[email protected]>
Signed-off-by Kees Cook <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Cc Christoph Lameter <[email protected]>
Cc Pekka Enberg <[email protected]>
Cc David Rientjes <[email protected]>
Cc Joonsoo Kim <[email protected]>
Cc <[email protected]>
Link http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook open here
Signed-off-by Linus Torvalds <[email protected]>

As previously promised, I've grabbed d5767057c9a7 ("uapi: rename ext2_swab() to swab() and share globally in swab.h") so that we'll have swab() on 4.19 and 4.14, but it wasn't enough.

There was another conflict with d36a63a943e3 ("kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED") which I've resolved by simply doing:

mm/slub.c +0 -0

index 958a8f7a3c253..d2db6bc5e788b 100644
@@ -248,7 +248,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
                                unsigned long ptr_addr)

{
#ifdef CONFIG_SLAB_FREELIST_HARDENED
- return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
+ return (void *)swab((unsigned long)ptr ^ s->random ^ ptr_addr);

#else
       return ptr;

#endif

Signature · 2 lines

Thanks,
Sasha

Kees Cook

2020-04-14 05:24 UTC #3 lore raw

Quoted 75 lines · Sasha Levin

Quoted 49 lines · On Fri, Apr 10, 2020 at 10:09:02AM +0200, [email protected] wrote:
The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation
Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak in that the ptr and ptr address were usually so close that the first XOR would result in an almost entirely 0-byte value[1], leaving most of the "secret" number ultimately being stored after the third XOR. A single blind memory content exposure of the freelist was generally sufficient to learn the secret.
Add a swab() call to mix bits a little more. This is a cheap way (1 cycle) to make attacks need more than a single exposure to learn the secret (or to know _where_ the exposure is in memory).
kmalloc-32 freelist walk, before:
ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...
after:
ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)
[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html
Quoted 11 lines
Fixes 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by Silvio Cesare <[email protected]>
Signed-off-by Kees Cook <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Cc Christoph Lameter <[email protected]>
Cc Pekka Enberg <[email protected]>
Cc David Rientjes <[email protected]>
Cc Joonsoo Kim <[email protected]>
Cc <[email protected]>
Link http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook open here
Signed-off-by Linus Torvalds <[email protected]>
As previously promised, I've grabbed d5767057c9a7 ("uapi: rename ext2_swab() to swab() and share globally in swab.h") so that we'll have swab() on 4.19 and 4.14, but it wasn't enough.
There was another conflict with d36a63a943e3 ("kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED") which I've resolved by simply doing:
mm/slub.c +0 -0
index 958a8f7a3c253..d2db6bc5e788b 100644

mm/slub.c static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr, · line 251

Quoted 5 lines

mm/slub.c +1 -1

@@ -248,7 +248,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
                                unsigned long ptr_addr)
 {
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-       return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
+       return (void *)swab((unsigned long)ptr ^ s->random ^ ptr_addr);

Eeek, no, no. The swab() must be on ptr_addr. I already sent a backport for this to stable, see:
https://lore.kernel.org/stable/202004131001.20346EB0E7@keescook

Please use that instead.

Signature · 1 line

Kees Cook

Greg KH

author maintainer

2020-04-14 08:09 UTC #4 lore raw

Quoted 86 lines · Kees Cook

Quoted 75 lines · Sasha Levin
Quoted 49 lines · On Fri, Apr 10, 2020 at 10:09:02AM +0200, [email protected] wrote:
The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 1ad53d9fa3f6168ebcf48a50e08b170432da2257 Mon Sep 17 00:00:00 2001
From: Kees Cook <[email protected]>
Date: Wed, 1 Apr 2020 21:04:23 -0700
Subject: [PATCH] slub: improve bit diffusion for freelist ptr obfuscation
Under CONFIG_SLAB_FREELIST_HARDENED=y, the obfuscation was relatively weak in that the ptr and ptr address were usually so close that the first XOR would result in an almost entirely 0-byte value[1], leaving most of the "secret" number ultimately being stored after the third XOR. A single blind memory content exposure of the freelist was generally sufficient to learn the secret.
Add a swab() call to mix bits a little more. This is a cheap way (1 cycle) to make attacks need more than a single exposure to learn the secret (or to know _where_ the exposure is in memory).
kmalloc-32 freelist walk, before:
ptr ptr_addr stored value secret
ffff90c22e019020@ffff90c22e019000 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019040@ffff90c22e019020 is 86528eb656b3b5fd (86528eb656b3b59d) ffff90c22e019060@ffff90c22e019040 is 86528eb656b3b5bd (86528eb656b3b59d) ffff90c22e019080@ffff90c22e019060 is 86528eb656b3b57d (86528eb656b3b59d) ffff90c22e0190a0@ffff90c22e019080 is 86528eb656b3b5bd (86528eb656b3b59d)
...
after:
ptr ptr_addr stored value secret
ffff9eed6e019020@ffff9eed6e019000 is 793d1135d52cda42 (86528eb656b3b59d) ffff9eed6e019040@ffff9eed6e019020 is 593d1135d52cda22 (86528eb656b3b59d) ffff9eed6e019060@ffff9eed6e019040 is 393d1135d52cda02 (86528eb656b3b59d) ffff9eed6e019080@ffff9eed6e019060 is 193d1135d52cdae2 (86528eb656b3b59d) ffff9eed6e0190a0@ffff9eed6e019080 is f93d1135d52cdac2 (86528eb656b3b59d)
[1] https://blog.infosectcbr.com.au/2020/03/weaknesses-in-linux-kernel-heap.html
Quoted 11 lines
Fixes 2482ddec670f ("mm: add SLUB free list pointer obfuscation")
Reported-by Silvio Cesare <[email protected]>
Signed-off-by Kees Cook <[email protected]>
Signed-off-by Andrew Morton <[email protected]>
Cc Christoph Lameter <[email protected]>
Cc Pekka Enberg <[email protected]>
Cc David Rientjes <[email protected]>
Cc Joonsoo Kim <[email protected]>
Cc <[email protected]>
Link http://lkml.kernel.org/r/202003051623.AF4F8CB@keescook open here
Signed-off-by Linus Torvalds <[email protected]>
As previously promised, I've grabbed d5767057c9a7 ("uapi: rename ext2_swab() to swab() and share globally in swab.h") so that we'll have swab() on 4.19 and 4.14, but it wasn't enough.
There was another conflict with d36a63a943e3 ("kasan, slub: fix more conflicts with CONFIG_SLAB_FREELIST_HARDENED") which I've resolved by simply doing:
mm/slub.c +0 -0
index 958a8f7a3c253..d2db6bc5e788b 100644
mm/slub.c static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr, · line 251
Quoted 5 lines
mm/slub.c +1 -1
@@ -248,7 +248,7 @@ static inline void *freelist_ptr(const struct kmem_cache *s, void *ptr,
                                unsigned long ptr_addr)
 {
 #ifdef CONFIG_SLAB_FREELIST_HARDENED
-       return (void *)((unsigned long)ptr ^ s->random ^ ptr_addr);
+       return (void *)swab((unsigned long)ptr ^ s->random ^ ptr_addr);
Eeek, no, no. The swab() must be on ptr_addr. I already sent a backport for this to stable, see:
https://lore.kernel.org/stable/202004131001.20346EB0E7@keescook

Quoted 1 line

Please use that instead.

I've dropped Sasha's backport and grabbed yours instead, thanks.

greg k-h