path: root/arch/x86/include/asm
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authorAndy Lutomirski <luto@kernel.org>2017-09-17 09:03:49 -0700
committerIngo Molnar <mingo@kernel.org>2017-09-17 18:59:08 +0200
commit52a2af400c1075219b3f0ce5c96fc961da44018a (patch)
tree0d51c712a3dbb9a18dd2adee5b786704fca48718 /arch/x86/include/asm
parent47061a24e2ee5bd8a40d473d47a5bd823fa0081f (diff)
x86/mm/64: Stop using CR3.PCID == 0 in ASID-aware code
Putting the logical ASID into CR3's PCID bits directly means that we have two cases to consider separately: ASID == 0 and ASID != 0. This means that bugs that only hit in one of these cases trigger nondeterministically. There were some bugs like this in the past, and I think there's still one in current kernels. In particular, we have a number of ASID-unware code paths that save CR3, write some special value, and then restore CR3. This includes suspend/resume, hibernate, kexec, EFI, and maybe other things I've missed. This is currently dangerous: if ASID != 0, then this code sequence will leave garbage in the TLB tagged for ASID 0. We could potentially see corruption when switching back to ASID 0. In principle, an initialize_tlbstate_and_flush() call after these sequences would solve the problem, but EFI, at least, does not call this. (And it probably shouldn't -- initialize_tlbstate_and_flush() is rather expensive.) Signed-off-by: Andy Lutomirski <luto@kernel.org> Cc: Borislav Petkov <bpetkov@suse.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/cdc14bbe5d3c3ef2a562be09a6368ffe9bd947a6.1505663533.git.luto@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/include/asm')
1 files changed, 19 insertions, 2 deletions
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index a999ba6b721f..c120b5db178a 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -286,14 +286,31 @@ static inline bool arch_vma_access_permitted(struct vm_area_struct *vma,
return __pkru_allows_pkey(vma_pkey(vma), write);
+ * If PCID is on, ASID-aware code paths put the ASID+1 into the PCID
+ * bits. This serves two purposes. It prevents a nasty situation in
+ * which PCID-unaware code saves CR3, loads some other value (with PCID
+ * == 0), and then restores CR3, thus corrupting the TLB for ASID 0 if
+ * the saved ASID was nonzero. It also means that any bugs involving
+ * loading a PCID-enabled CR3 with CR4.PCIDE off will trigger
+ * deterministically.
+ */
static inline unsigned long build_cr3(struct mm_struct *mm, u16 asid)
- return __sme_pa(mm->pgd) | asid;
+ if (static_cpu_has(X86_FEATURE_PCID)) {
+ VM_WARN_ON_ONCE(asid > 4094);
+ return __sme_pa(mm->pgd) | (asid + 1);
+ } else {
+ VM_WARN_ON_ONCE(asid != 0);
+ return __sme_pa(mm->pgd);
+ }
static inline unsigned long build_cr3_noflush(struct mm_struct *mm, u16 asid)
- return __sme_pa(mm->pgd) | asid | CR3_NOFLUSH;
+ VM_WARN_ON_ONCE(asid > 4094);
+ return __sme_pa(mm->pgd) | (asid + 1) | CR3_NOFLUSH;

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