FAILED: patch "[PATCH] x86/mm/pat: fix VM_PAT handling in COW mappings" failed to apply to 5.4-stable tree

The patch below does not apply to the 5.4-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]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.4.y
git checkout FETCH_HEAD
git cherry-pick -x 04c35ab3bdae7fefbd7c7a7355f29fa03a035221
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2024040847-departure-lining-fed7@gregkh' --subject-prefix 'PATCH 5.4.y' HEAD^..

Possible dependencies:

04c35ab3bdae ("x86/mm/pat: fix VM_PAT handling in COW mappings")

thanks,

greg k-h

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

From 04c35ab3bdae7fefbd7c7a7355f29fa03a035221 Mon Sep 17 00:00:00 2001
From: David Hildenbrand <[email protected]>
Date: Wed, 3 Apr 2024 23:21:30 +0200
Subject: [PATCH] x86/mm/pat: fix VM_PAT handling in COW mappings

PAT handling won't do the right thing in COW mappings: the first PTE (or, in fact, all PTEs) can be replaced during write faults to point at anon folios. Reliably recovering the correct PFN and cachemode using follow_phys() from PTEs will not work in COW mappings.

Using follow_phys(), we might just get the address+protection of the anon folio (which is very wrong), or fail on swap/nonswap entries, failing follow_phys() and triggering a WARN_ON_ONCE() in untrack_pfn() and track_pfn_copy(), not properly calling free_pfn_range().

In free_pfn_range(), we either wouldn't call memtype_free() or would call it with the wrong range, possibly leaking memory.

To fix that, let's update follow_phys() to refuse returning anon folios, and fallback to using the stored PFN inside vma->vm_pgoff for COW mappings if we run into that.

We will now properly handle untrack_pfn() with COW mappings, where we don't need the cachemode. We'll have to fail fork()->track_pfn_copy() if the first page was replaced by an anon folio, though: we'd have to store the cachemode in the VMA to make this work, likely growing the VMA size.

For now, lets keep it simple and let track_pfn_copy() just fail in that
case: it would have failed in the past with swap/nonswap entries already, and it would have done the wrong thing with anon folios.

Simple reproducer to trigger the WARN_ON_ONCE() in untrack_pfn():

<--- C reproducer --->
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <liburing.h>

int main(void)

 {
         struct io_uring_params p = {};
         int ring_fd;
         size_t size;
         char *map;

         ring_fd = io_uring_setup(1, &p);
         if (ring_fd < 0) {
                 perror("io_uring_setup");
                 return 1;
         }
         size = p.sq_off.array + p.sq_entries * sizeof(unsigned);

         /* Map the submission queue ring MAP_PRIVATE */
         map = mmap(0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE,
                    ring_fd, IORING_OFF_SQ_RING);
         if (map == MAP_FAILED) {
                 perror("mmap");
                 return 1;
         }

         /* We have at least one page. Let's COW it. */
         *map = 0;
         pause();
         return 0;

}
<--- C reproducer --->

On a system with 16 GiB RAM and swap configured:

 # ./iouring &
 # memhog 16G
 # killall iouring
[  301.552930] ------------[ cut here ]------------
[  301.553285] WARNING: CPU: 7 PID: 1402 at arch/x86/mm/pat/memtype.c:1060 untrack_pfn+0xf4/0x100
[  301.553989] Modules linked in: binfmt_misc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_g
[  301.558232] CPU: 7 PID: 1402 Comm: iouring Not tainted 6.7.5-100.fc38.x86_64 #1
[  301.558772] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebu4
[  301.559569] RIP: 0010:untrack_pfn+0xf4/0x100
[  301.559893] Code: 75 c4 eb cf 48 8b 43 10 8b a8 e8 00 00 00 3b 6b 28 74 b8 48 8b 7b 30 e8 ea 1a f7 000
[  301.561189] RSP: 0018:ffffba2c0377fab8 EFLAGS: 00010282
[  301.561590] RAX: 00000000ffffffea RBX: ffff9208c8ce9cc0 RCX: 000000010455e047
[  301.562105] RDX: 07fffffff0eb1e0a RSI: 0000000000000000 RDI: ffff9208c391d200
[  301.562628] RBP: 0000000000000000 R08: ffffba2c0377fab8 R09: 0000000000000000
[  301.563145] R10: ffff9208d2292d50 R11: 0000000000000002 R12: 00007fea890e0000
[  301.563669] R13: 0000000000000000 R14: ffffba2c0377fc08 R15: 0000000000000000
[  301.564186] FS:  0000000000000000(0000) GS:ffff920c2fbc0000(0000) knlGS:0000000000000000
[  301.564773] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  301.565197] CR2: 00007fea88ee8a20 CR3: 00000001033a8000 CR4: 0000000000750ef0
[  301.565725] PKRU: 55555554
[  301.565944] Call Trace:
[  301.566148]  <TASK>
[  301.566325]  ? untrack_pfn+0xf4/0x100
[  301.566618]  ? __warn+0x81/0x130
[  301.566876]  ? untrack_pfn+0xf4/0x100
[  301.567163]  ? report_bug+0x171/0x1a0
[  301.567466]  ? handle_bug+0x3c/0x80
[  301.567743]  ? exc_invalid_op+0x17/0x70
[  301.568038]  ? asm_exc_invalid_op+0x1a/0x20
[  301.568363]  ? untrack_pfn+0xf4/0x100
[  301.568660]  ? untrack_pfn+0x65/0x100
[  301.568947]  unmap_single_vma+0xa6/0xe0
[  301.569247]  unmap_vmas+0xb5/0x190
[  301.569532]  exit_mmap+0xec/0x340
[  301.569801]  __mmput+0x3e/0x130
[  301.570051]  do_exit+0x305/0xaf0
...

index 0d72183b5dd0..36b603d0cdde 100644
@@ -947,6 +947,38 @@ static void free_pfn_range(u64 paddr, unsigned long size)
 		memtype_free(paddr, paddr + size);
 }
 
+static int get_pat_info(struct vm_area_struct *vma, resource_size_t *paddr,
+		pgprot_t *pgprot)
+{
+	unsigned long prot;
+
+	VM_WARN_ON_ONCE(!(vma->vm_flags & VM_PAT));
+
+	/*
+	 * We need the starting PFN and cachemode used for track_pfn_remap()
+	 * that covered the whole VMA. For most mappings, we can obtain that
+	 * information from the page tables. For COW mappings, we might now
+	 * suddenly have anon folios mapped and follow_phys() will fail.
+	 *
+	 * Fallback to using vma->vm_pgoff, see remap_pfn_range_notrack(), to
+	 * detect the PFN. If we need the cachemode as well, we're out of luck
+	 * for now and have to fail fork().
+	 */
+	if (!follow_phys(vma, vma->vm_start, 0, &prot, paddr)) {
+		if (pgprot)
+			*pgprot = __pgprot(prot);
+		return 0;
+	}
+	if (is_cow_mapping(vma->vm_flags)) {
+		if (pgprot)
+			return -EINVAL;
+		*paddr = (resource_size_t)vma->vm_pgoff << PAGE_SHIFT;
+		return 0;
+	}
+	WARN_ON_ONCE(1);
+	return -EINVAL;
+}
+
 /*
  * track_pfn_copy is called when vma that is covering the pfnmap gets
  * copied through copy_page_range().

@@ -957,20 +989,13 @@ static void free_pfn_range(u64 paddr, unsigned long size)
 int track_pfn_copy(struct vm_area_struct *vma)
 {
 	resource_size_t paddr;
-	unsigned long prot;
 	unsigned long vma_size = vma->vm_end - vma->vm_start;
 	pgprot_t pgprot;
 
 	if (vma->vm_flags & VM_PAT) {
-		/*
-		 * reserve the whole chunk covered by vma. We need the
-		 * starting address and protection from pte.
-		 */
-		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
-			WARN_ON_ONCE(1);
+		if (get_pat_info(vma, &paddr, &pgprot))
 			return -EINVAL;
-		}
-		pgprot = __pgprot(prot);
+		/* reserve the whole chunk covered by vma. */
 		return reserve_pfn_range(paddr, vma_size, &pgprot, 1);
 	}
 

@@ -1045,7 +1070,6 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 		 unsigned long size, bool mm_wr_locked)
 {
 	resource_size_t paddr;
-	unsigned long prot;
 
 	if (vma && !(vma->vm_flags & VM_PAT))
 		return;

@@ -1053,11 +1077,8 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 	/* free the chunk starting from pfn or the whole chunk */
 	paddr = (resource_size_t)pfn << PAGE_SHIFT;
 	if (!paddr && !size) {
-		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
-			WARN_ON_ONCE(1);
+		if (get_pat_info(vma, &paddr, NULL))
 			return;
-		}
-
 		size = vma->vm_end - vma->vm_start;
 	}
 	free_pfn_range(paddr, size);

index 904f70b99498..d2155ced45f8 100644
@@ -5973,6 +5973,10 @@ int follow_phys(struct vm_area_struct *vma,
 		goto out;
 	pte = ptep_get(ptep);
 
+	/* Never return PFNs of anon folios in COW mappings. */
+	if (vm_normal_folio(vma, address, pte))
+		goto unlock;
+
 	if ((flags & FOLL_WRITE) && !pte_write(pte))
 		goto unlock;
 

PAT handling won't do the right thing in COW mappings: the first PTE (or, in fact, all PTEs) can be replaced during write faults to point at anon folios. Reliably recovering the correct PFN and cachemode using follow_phys() from PTEs will not work in COW mappings.

Using follow_phys(), we might just get the address+protection of the anon folio (which is very wrong), or fail on swap/nonswap entries, failing follow_phys() and triggering a WARN_ON_ONCE() in untrack_pfn() and track_pfn_copy(), not properly calling free_pfn_range().

In free_pfn_range(), we either wouldn't call memtype_free() or would call it with the wrong range, possibly leaking memory.

To fix that, let's update follow_phys() to refuse returning anon folios, and fallback to using the stored PFN inside vma->vm_pgoff for COW mappings if we run into that.

We will now properly handle untrack_pfn() with COW mappings, where we don't need the cachemode. We'll have to fail fork()->track_pfn_copy() if the first page was replaced by an anon folio, though: we'd have to store the cachemode in the VMA to make this work, likely growing the VMA size.

For now, lets keep it simple and let track_pfn_copy() just fail in that
case: it would have failed in the past with swap/nonswap entries already, and it would have done the wrong thing with anon folios.

Simple reproducer to trigger the WARN_ON_ONCE() in untrack_pfn():

<--- C reproducer --->
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>
#include <liburing.h>

int main(void)

 {
         struct io_uring_params p = {};
         int ring_fd;
         size_t size;
         char *map;

         ring_fd = io_uring_setup(1, &p);
         if (ring_fd < 0) {
                 perror("io_uring_setup");
                 return 1;
         }
         size = p.sq_off.array + p.sq_entries * sizeof(unsigned);

         /* Map the submission queue ring MAP_PRIVATE */
         map = mmap(0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE,
                    ring_fd, IORING_OFF_SQ_RING);
         if (map == MAP_FAILED) {
                 perror("mmap");
                 return 1;
         }

         /* We have at least one page. Let's COW it. */
         *map = 0;
         pause();
         return 0;

}
<--- C reproducer --->

On a system with 16 GiB RAM and swap configured:

 # ./iouring &
 # memhog 16G
 # killall iouring
[  301.552930] ------------[ cut here ]------------
[  301.553285] WARNING: CPU: 7 PID: 1402 at arch/x86/mm/pat/memtype.c:1060 untrack_pfn+0xf4/0x100
[  301.553989] Modules linked in: binfmt_misc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_g
[  301.558232] CPU: 7 PID: 1402 Comm: iouring Not tainted 6.7.5-100.fc38.x86_64 #1
[  301.558772] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebu4
[  301.559569] RIP: 0010:untrack_pfn+0xf4/0x100
[  301.559893] Code: 75 c4 eb cf 48 8b 43 10 8b a8 e8 00 00 00 3b 6b 28 74 b8 48 8b 7b 30 e8 ea 1a f7 000
[  301.561189] RSP: 0018:ffffba2c0377fab8 EFLAGS: 00010282
[  301.561590] RAX: 00000000ffffffea RBX: ffff9208c8ce9cc0 RCX: 000000010455e047
[  301.562105] RDX: 07fffffff0eb1e0a RSI: 0000000000000000 RDI: ffff9208c391d200
[  301.562628] RBP: 0000000000000000 R08: ffffba2c0377fab8 R09: 0000000000000000
[  301.563145] R10: ffff9208d2292d50 R11: 0000000000000002 R12: 00007fea890e0000
[  301.563669] R13: 0000000000000000 R14: ffffba2c0377fc08 R15: 0000000000000000
[  301.564186] FS:  0000000000000000(0000) GS:ffff920c2fbc0000(0000) knlGS:0000000000000000
[  301.564773] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  301.565197] CR2: 00007fea88ee8a20 CR3: 00000001033a8000 CR4: 0000000000750ef0
[  301.565725] PKRU: 55555554
[  301.565944] Call Trace:
[  301.566148]  <TASK>
[  301.566325]  ? untrack_pfn+0xf4/0x100
[  301.566618]  ? __warn+0x81/0x130
[  301.566876]  ? untrack_pfn+0xf4/0x100
[  301.567163]  ? report_bug+0x171/0x1a0
[  301.567466]  ? handle_bug+0x3c/0x80
[  301.567743]  ? exc_invalid_op+0x17/0x70
[  301.568038]  ? asm_exc_invalid_op+0x1a/0x20
[  301.568363]  ? untrack_pfn+0xf4/0x100
[  301.568660]  ? untrack_pfn+0x65/0x100
[  301.568947]  unmap_single_vma+0xa6/0xe0
[  301.569247]  unmap_vmas+0xb5/0x190
[  301.569532]  exit_mmap+0xec/0x340
[  301.569801]  __mmput+0x3e/0x130
[  301.570051]  do_exit+0x305/0xaf0
...

---
 arch/x86/mm/pat.c | 50 ++++++++++++++++++++++++++++++++++-------------
 mm/memory.c       |  4 ++++
 2 files changed, 40 insertions(+), 14 deletions(-)

index c7c4e2f8c6a5..39d1c35e7491 100644
@@ -34,6 +34,7 @@
 
 #include "pat_internal.h"
 #include "mm_internal.h"
+#include "../../mm/internal.h"	/* is_cow_mapping() */
 
 #undef pr_fmt
 #define pr_fmt(fmt) "" fmt

@@ -955,6 +956,38 @@ static void free_pfn_range(u64 paddr, unsigned long size)
 		free_memtype(paddr, paddr + size);
 }
 
+static int get_pat_info(struct vm_area_struct *vma, resource_size_t *paddr,
+		pgprot_t *pgprot)
+{
+	unsigned long prot;
+
+	VM_WARN_ON_ONCE(!(vma->vm_flags & VM_PAT));
+
+	/*
+	 * We need the starting PFN and cachemode used for track_pfn_remap()
+	 * that covered the whole VMA. For most mappings, we can obtain that
+	 * information from the page tables. For COW mappings, we might now
+	 * suddenly have anon folios mapped and follow_phys() will fail.
+	 *
+	 * Fallback to using vma->vm_pgoff, see remap_pfn_range_notrack(), to
+	 * detect the PFN. If we need the cachemode as well, we're out of luck
+	 * for now and have to fail fork().
+	 */
+	if (!follow_phys(vma, vma->vm_start, 0, &prot, paddr)) {
+		if (pgprot)
+			*pgprot = __pgprot(prot);
+		return 0;
+	}
+	if (is_cow_mapping(vma->vm_flags)) {
+		if (pgprot)
+			return -EINVAL;
+		*paddr = (resource_size_t)vma->vm_pgoff << PAGE_SHIFT;
+		return 0;
+	}
+	WARN_ON_ONCE(1);
+	return -EINVAL;
+}
+
 /*
  * track_pfn_copy is called when vma that is covering the pfnmap gets
  * copied through copy_page_range().

@@ -965,20 +998,13 @@ static void free_pfn_range(u64 paddr, unsigned long size)
 int track_pfn_copy(struct vm_area_struct *vma)
 {
 	resource_size_t paddr;
-	unsigned long prot;
 	unsigned long vma_size = vma->vm_end - vma->vm_start;
 	pgprot_t pgprot;
 
 	if (vma->vm_flags & VM_PAT) {
-		/*
-		 * reserve the whole chunk covered by vma. We need the
-		 * starting address and protection from pte.
-		 */
-		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
-			WARN_ON_ONCE(1);
+		if (get_pat_info(vma, &paddr, &pgprot))
 			return -EINVAL;
-		}
-		pgprot = __pgprot(prot);
+		/* reserve the whole chunk covered by vma. */
 		return reserve_pfn_range(paddr, vma_size, &pgprot, 1);
 	}
 

@@ -1053,7 +1079,6 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 		 unsigned long size)
 {
 	resource_size_t paddr;
-	unsigned long prot;
 
 	if (vma && !(vma->vm_flags & VM_PAT))
 		return;

@@ -1061,11 +1086,8 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
 	/* free the chunk starting from pfn or the whole chunk */
 	paddr = (resource_size_t)pfn << PAGE_SHIFT;
 	if (!paddr && !size) {
-		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
-			WARN_ON_ONCE(1);
+		if (get_pat_info(vma, &paddr, NULL))
 			return;
-		}
-
 		size = vma->vm_end - vma->vm_start;
 	}
 	free_pfn_range(paddr, size);

index d514f69d9717..f8d76c66311d 100644
@@ -4428,6 +4428,10 @@ int follow_phys(struct vm_area_struct *vma,
 		goto out;
 	pte = *ptep;
 
+	/* Never return PFNs of anon folios in COW mappings. */
+	if (vm_normal_page(vma, address, pte))
+		goto unlock;
+
 	if ((flags & FOLL_WRITE) && !pte_write(pte))
 		goto unlock;
 

2.44.0

Backport notes:
(1) #include "../../mm/internal.h" to make it compile
(2) vm_normal_folio -> vm_normal_page
(3) follow_phys() is also used in generic_access_phys(), where we would now also reject anon pages. I don't think we care.

Cheers,

David / dhildenb