Commit 36639013 authored Jul 25, 2024 by Will Deacon

arm64: mm: Fix lockless walks with static and dynamic page-table folding

Lina reports random oopsen originating from the fast GUP code when
16K pages are used with 4-level page-tables, the fourth level being
folded at runtime due to lack of LPA2.

In this configuration, the generic implementation of
p4d_offset_lockless() will return a 'p4d_t *' corresponding to the
'pgd_t' allocated on the stack of the caller, gup_fast_pgd_range().
This is normally fine, but when the fourth level of page-table is folded
at runtime, pud_offset_lockless() will offset from the address of the
'p4d_t' to calculate the address of the PUD in the same page-table page.
This results in a stray stack read when the 'p4d_t' has been allocated
on the stack and can send the walker into the weeds.

Fix the problem by providing our own definition of p4d_offset_lockless()
when CONFIG_PGTABLE_LEVELS <= 4 which returns the real page-table
pointer rather than the address of the local stack variable.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/50360968-13fb-4e6f-8f52-1725b3177215@asahilina.net


Fixes: 0dd4f60a ("arm64: mm: Add support for folding PUDs at runtime")
Reported-by: Asahi Lina <lina@asahilina.net>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20240725090345.28461-1-will@kernel.org


Signed-off-by: Will Deacon <will@kernel.org>

parent f3dfcd25

arch/arm64/include/asm/pgtable.h

+22 −0

Original line number	Diff line number	Diff line
		@@ -1065,6 +1065,28 @@ static inline bool pgtable_l5_enabled(void) { return false; }

		#define p4d_offset_kimg(dir,addr) ((p4d_t *)dir)

		static inline
		p4d_t p4d_offset_lockless_folded(pgd_t pgdp, pgd_t pgd, unsigned long addr)
		{
		/*
		* With runtime folding of the pud, pud_offset_lockless() passes
		* the 'pgd_t *' we return here to p4d_to_folded_pud(), which
		* will offset the pointer assuming that it points into
		* a page-table page. However, the fast GUP path passes us a
		* pgd_t allocated on the stack and so we must use the original
		* pointer in 'pgdp' to construct the p4d pointer instead of
		* using the generic p4d_offset_lockless() implementation.
		*
		* Note: reusing the original pointer means that we may
		* dereference the same (live) page-table entry multiple times.
		* This is safe because it is still only loaded once in the
		* context of each level and the CPU guarantees same-address
		* read-after-read ordering.
		*/
		return p4d_offset(pgdp, addr);
		}
		#define p4d_offset_lockless p4d_offset_lockless_folded

		#endif /* CONFIG_PGTABLE_LEVELS > 4 */

		#define pgd_ERROR(e) \