x86/pti: Make unpoison of pgd for trusted boot work for real
Subsystems (3)
PATCH 1/1 x86/pti: Make unpoison of pgd for trusted boot work for real
From: Dave Hansen <[email protected]>
commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream
The inital fix for trusted boot and PTI potentially misses the pgd clearing if pud_alloc() sets a PGD. It probably works in *practice* because for two adjacent calls to map_tboot_page() that share a PGD entry, the first will clear NX, *then* allocate and set the PGD (without NX clear). The second call will *not* allocate but will clear the NX bit.
Defer the NX clearing to a point after it is known that all top-level allocations have occurred. Add a comment to clarify why.
[ tglx: Massaged changelog ]
hughd notes: I have not tested tboot, but this looks to me as necessary and as safe in old-Kaiser backports as it is upstream; I'm not submitting the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e, and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.
--- arch/x86/kernel/tboot.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
arch/x86/kernel/tboot.c +10 -0
@@ -134,6 +134,16 @@ static int map_tboot_page(unsigned long vaddr, unsigned long pfn,return -1 ;set_pte_at (&tboot_mm, vaddr, pte,pfn_pte (pfn, prot));pte_unmap (pte); + +/* +* PTI poisons low addresses in the kernel page tables in the +* name of making them unusable for userspace. To execute +* code at such a low address, the poison must be cleared. +* +* Note: 'pgd' actually gets set in pud_alloc(). +*/ + pgd->pgd &= ~_PAGE_NX; +return 0 ; }
Signature · 1 line
2.16.0.rc1.238.g530d649a79-goog
Patch "x86/pti: Make unpoison of pgd for trusted boot work for real" has been added to the 4.9-stable tree
This is a note to let you know that I've just added the patch titled
x86/pti: Make unpoison of pgd for trusted boot work for real
to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-pti-make-unpoison-of-pgd-for-trusted-boot-work-for-real.patch
and it can be found in the queue-4.9 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.
Quoted 1 line
From [email protected] Mon Feb 5 04:59:38 2018
From: Hugh Dickins <[email protected]>
Date: Mon, 29 Jan 2018 18:17:26 -0800
Subject: x86/pti: Make unpoison of pgd for trusted boot work for real
To: Greg Kroah-Hartman <[email protected]>
Cc: Dave Hansen <[email protected]>, Thomas Gleixner <[email protected]>, Jon Masters <[email protected]>, Tim Chen <[email protected]>, [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], Jiri Kosina <[email protected]>, Hugh Dickins <[email protected]>
Message-ID: <[email protected]>
From: Dave Hansen <[email protected]>
commit 445b69e3b75e42362a5bdc13c8b8f61599e2228a upstream
The inital fix for trusted boot and PTI potentially misses the pgd clearing if pud_alloc() sets a PGD. It probably works in *practice* because for two adjacent calls to map_tboot_page() that share a PGD entry, the first will clear NX, *then* allocate and set the PGD (without NX clear). The second call will *not* allocate but will clear the NX bit.
Defer the NX clearing to a point after it is known that all top-level allocations have occurred. Add a comment to clarify why.
[ tglx: Massaged changelog ]
[ hughd notes: I have not tested tboot, but this looks to me as necessary and as safe in old-Kaiser backports as it is upstream; I'm not submitting the commit-to-be-fixed 262b6b30087, since it was undone by 445b69e3b75e, and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.]
--- arch/x86/kernel/tboot.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
arch/x86/kernel/tboot.c +10 -0
@@ -134,6 +134,16 @@ static int map_tboot_page(unsigned longreturn -1 ;set_pte_at (&tboot_mm, vaddr, pte,pfn_pte (pfn, prot));pte_unmap (pte); + +/* +* PTI poisons low addresses in the kernel page tables in the +* name of making them unusable for userspace. To execute +* code at such a low address, the poison must be cleared. +* +* Note: 'pgd' actually gets set in pud_alloc(). +*/ + pgd->pgd &= ~_PAGE_NX; +return 0 ; }
Patches currently in stable-queue which might be from [email protected] are
queue-4.9/kaiser-allocate-pgd-with-order-0-when-pti-off.patch queue-4.9/x86-pti-make-unpoison-of-pgd-for-trusted-boot-work-for-real.patch queue-4.9/kaiser-fix-intel_bts-perf-crashes.patch