From: John Hubbard <jhubbard@nvidia.com> To: Andrew Morton <akpm@linux-foundation.org> Cc: Albert Ou <aou@eecs.berkeley.edu>, Alexandre Ghiti <alexghiti@rivosinc.com>, Andrew Jones <ajones@ventanamicro.com>, "Hugh Dickins" <hughd@google.com>, Palmer Dabbelt <palmer@dabbelt.com>, "Paul Walmsley" <paul.walmsley@sifive.com>, Qinglin Pan <panqinglin2020@iscas.ac.cn>, <linux-riscv@lists.infradead.org>, <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, John Hubbard <jhubbard@nvidia.com>, James Houghton <jthoughton@google.com>, Ryan Roberts <ryan.roberts@arm.com> Subject: [PATCH] mm: riscv: fix an unsafe pte read in huge_pte_alloc() Date: Mon, 3 Jul 2023 12:00:44 -0700 [thread overview] Message-ID: <20230703190044.311730-1-jhubbard@nvidia.com> (raw) The WARN_ON_ONCE() statement in riscv's huge_pte_alloc() is susceptible to false positives, because the pte is read twice at the C language level, locklessly, within the same conditional statement. Depending on compiler behavior, this can lead to generated machine code that actually reads the pte just once, or twice. Reading twice will expose the code to changing pte values and cause incorrect behavior. In [1], similar code actually caused a kernel crash on 64-bit x86, when using clang to build the kernel, but only after the conversion from *pte reads, to ptep_get(pte). The latter uses READ_ONCE(), which forced a double read of *pte. Rather than waiting for the upcoming ptep_get() conversion, just convert this part of the code now, but in a way that avoids the above problem: take a single snapshot of the pte before using it in the WARN conditional. As expected, this preparatory step does not actually change the generated code ("make mm/hugetlbpage.s"), on riscv64, when using a gcc 12.2 cross compiler. [1] https://lore.kernel.org/20230630013203.1955064-1-jhubbard@nvidia.com Suggested-by: James Houghton <jthoughton@google.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- arch/riscv/mm/hugetlbpage.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/arch/riscv/mm/hugetlbpage.c b/arch/riscv/mm/hugetlbpage.c index 542883b3b49b..96225a8533ad 100644 --- a/arch/riscv/mm/hugetlbpage.c +++ b/arch/riscv/mm/hugetlbpage.c @@ -73,7 +73,11 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, } out: - WARN_ON_ONCE(pte && pte_present(*pte) && !pte_huge(*pte)); + if (pte) { + pte_t pteval = ptep_get_lockless(pte); + + WARN_ON_ONCE(pte_present(pteval) && !pte_huge(pteval)); + } return pte; } base-commit: 0a8d6c9c7128a93689fba384cdd7f72b0ce19abd -- 2.41.0
WARNING: multiple messages have this Message-ID (diff)
From: John Hubbard <jhubbard@nvidia.com> To: Andrew Morton <akpm@linux-foundation.org> Cc: Albert Ou <aou@eecs.berkeley.edu>, Alexandre Ghiti <alexghiti@rivosinc.com>, Andrew Jones <ajones@ventanamicro.com>, "Hugh Dickins" <hughd@google.com>, Palmer Dabbelt <palmer@dabbelt.com>, "Paul Walmsley" <paul.walmsley@sifive.com>, Qinglin Pan <panqinglin2020@iscas.ac.cn>, <linux-riscv@lists.infradead.org>, <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>, John Hubbard <jhubbard@nvidia.com>, James Houghton <jthoughton@google.com>, Ryan Roberts <ryan.roberts@arm.com> Subject: [PATCH] mm: riscv: fix an unsafe pte read in huge_pte_alloc() Date: Mon, 3 Jul 2023 12:00:44 -0700 [thread overview] Message-ID: <20230703190044.311730-1-jhubbard@nvidia.com> (raw) The WARN_ON_ONCE() statement in riscv's huge_pte_alloc() is susceptible to false positives, because the pte is read twice at the C language level, locklessly, within the same conditional statement. Depending on compiler behavior, this can lead to generated machine code that actually reads the pte just once, or twice. Reading twice will expose the code to changing pte values and cause incorrect behavior. In [1], similar code actually caused a kernel crash on 64-bit x86, when using clang to build the kernel, but only after the conversion from *pte reads, to ptep_get(pte). The latter uses READ_ONCE(), which forced a double read of *pte. Rather than waiting for the upcoming ptep_get() conversion, just convert this part of the code now, but in a way that avoids the above problem: take a single snapshot of the pte before using it in the WARN conditional. As expected, this preparatory step does not actually change the generated code ("make mm/hugetlbpage.s"), on riscv64, when using a gcc 12.2 cross compiler. [1] https://lore.kernel.org/20230630013203.1955064-1-jhubbard@nvidia.com Suggested-by: James Houghton <jthoughton@google.com> Cc: Ryan Roberts <ryan.roberts@arm.com> Signed-off-by: John Hubbard <jhubbard@nvidia.com> --- arch/riscv/mm/hugetlbpage.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-) diff --git a/arch/riscv/mm/hugetlbpage.c b/arch/riscv/mm/hugetlbpage.c index 542883b3b49b..96225a8533ad 100644 --- a/arch/riscv/mm/hugetlbpage.c +++ b/arch/riscv/mm/hugetlbpage.c @@ -73,7 +73,11 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, } out: - WARN_ON_ONCE(pte && pte_present(*pte) && !pte_huge(*pte)); + if (pte) { + pte_t pteval = ptep_get_lockless(pte); + + WARN_ON_ONCE(pte_present(pteval) && !pte_huge(pteval)); + } return pte; } base-commit: 0a8d6c9c7128a93689fba384cdd7f72b0ce19abd -- 2.41.0 _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv
next reply other threads:[~2023-07-03 19:01 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-07-03 19:00 John Hubbard [this message] 2023-07-03 19:00 ` [PATCH] mm: riscv: fix an unsafe pte read in huge_pte_alloc() John Hubbard 2023-07-04 6:01 ` Andrew Jones 2023-07-04 6:01 ` Andrew Jones 2023-07-04 7:06 ` Ryan Roberts 2023-07-04 7:06 ` Ryan Roberts 2023-07-05 23:38 ` Palmer Dabbelt 2023-07-05 23:38 ` Palmer Dabbelt 2023-07-05 23:50 ` patchwork-bot+linux-riscv 2023-07-05 23:50 ` patchwork-bot+linux-riscv
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