x86/pti: Make unpoison of pgd for trusted boot work for real
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
@@ -140,6 +140,16 @@ 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
Greg Kroah-Hartman
maintainerQuoted 38 lines · Hugh Dickins
From: Dave Hansen <[email protected]>
commit
445b69e3b75e42362a5bdc13c8b8f61599e2228aupstreamThe 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 tomap_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 ]
Fixes 262b6b30087 ("x86/tboot: Unbreak tboot with PTI enabled")Signed-off-by Dave Hansen <[email protected]>Signed-off-by Thomas Gleixner <[email protected]>Reviewed-by Andrea Arcangeli <[email protected]>Cc Jon Masters <[email protected]>Cc Tim Chen <[email protected]>Cc Jiri Kosina <[email protected]>Signed-off-by Hugh Dickins <[email protected]>
Quoted 4 lines
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 by445b69e3b75e, and makes conflict trouble because of 5-level's p4d versus 4-level's pgd.
Thanks for the backport, now queued up.
greg k-h