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x86/mm: Fix pti_clone_pgtable() alignment assumption 

[ Upstream commit 41e71dbb0e0a0fe214545fe64af031303a08524c ]

Guenter reported dodgy crashes on an i386-nosmp build using GCC-11
that had the form of endless traps until entry stack exhaust and then
#DF from the stack guard.

It turned out that pti_clone_pgtable() had alignment assumptions on
the start address, notably it hard assumes start is PMD aligned. This
is true on x86_64, but very much not true on i386.

These assumptions can cause the end condition to malfunction, leading
to a 'short' clone. Guess what happens when the user mapping has a
short copy of the entry text?

Use the correct increment form for addr to avoid alignment
assumptions.

Fixes: 16a3fe634f ("x86/mm/pti: Clone kernel-image on PTE level for 32 bit")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20240731163105.GG33588@noisy.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Peter Zijlstra 2024-07-31 18:31:05 +02:00 committed by Greg Kroah-Hartman
parent c7b46f69d1
commit ca07aab70d
1 changed files with 3 additions and 3 deletions
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6
arch/x86/mm/pti.c
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@ -374,14 +374,14 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
*/ */
*target_pmd = *pmd; *target_pmd = *pmd;
addr += PMD_SIZE; addr = round_up(addr + 1, PMD_SIZE);
} else if (level == PTI_CLONE_PTE) { } else if (level == PTI_CLONE_PTE) {
/* Walk the page-table down to the pte level */ /* Walk the page-table down to the pte level */
pte = pte_offset_kernel(pmd, addr); pte = pte_offset_kernel(pmd, addr);
if (pte_none(*pte)) { if (pte_none(*pte)) {
addr += PAGE_SIZE; addr = round_up(addr + 1, PAGE_SIZE);
continue; continue;
} }
@ -401,7 +401,7 @@ pti_clone_pgtable(unsigned long start, unsigned long end,
/* Clone the PTE */ /* Clone the PTE */
*target_pte = *pte; *target_pte = *pte;
addr += PAGE_SIZE; addr = round_up(addr + 1, PAGE_SIZE);
} else { } else {
BUG(); BUG();