From ae397b57a0e32671a089686cf442ab0b87f7d4fd Mon Sep 17 00:00:00 2001 From: Matt Redfearn Date: Tue, 17 Apr 2018 16:40:00 +0100 Subject: [PATCH] MIPS: memset.S: Fix clobber of v1 in last_fixup [ Upstream commit c96eebf07692e53bf4dd5987510d8b550e793598 ] The label .Llast_fixup\@ is jumped to on page fault within the final byte set loop of memset (on < MIPSR6 architectures). For some reason, in this fault handler, the v1 register is randomly set to a2 & STORMASK. This clobbers v1 for the calling function. This can be observed with the following test code: static int __init __attribute__((optimize("O0"))) test_clear_user(void) { register int t asm("v1"); char *test; int j, k; pr_info("\n\n\nTesting clear_user\n"); test = vmalloc(PAGE_SIZE); for (j = 256; j < 512; j++) { t = 0xa5a5a5a5; if ((k = clear_user(test + PAGE_SIZE - 256, j)) != j - 256) { pr_err("clear_user (%px %d) returned %d\n", test + PAGE_SIZE - 256, j, k); } if (t != 0xa5a5a5a5) { pr_err("v1 was clobbered to 0x%x!\n", t); } } return 0; } late_initcall(test_clear_user); Which demonstrates that v1 is indeed clobbered (MIPS64): Testing clear_user v1 was clobbered to 0x1! v1 was clobbered to 0x2! v1 was clobbered to 0x3! v1 was clobbered to 0x4! v1 was clobbered to 0x5! v1 was clobbered to 0x6! v1 was clobbered to 0x7! Since the number of bytes that could not be set is already contained in a2, the andi placing a value in v1 is not necessary and actively harmful in clobbering v1. Reported-by: James Hogan Signed-off-by: Matt Redfearn Cc: Ralf Baechle Cc: linux-mips@linux-mips.org Cc: stable@vger.kernel.org Patchwork: https://patchwork.linux-mips.org/patch/19109/ Signed-off-by: James Hogan Signed-off-by: Sasha Levin --- arch/mips/lib/memset.S | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/mips/lib/memset.S b/arch/mips/lib/memset.S index 3517faab7107c..cda33475a4813 100644 --- a/arch/mips/lib/memset.S +++ b/arch/mips/lib/memset.S @@ -255,7 +255,7 @@ .Llast_fixup\@: jr ra - andi v1, a2, STORMASK + nop .Lsmall_fixup\@: PTR_SUBU a2, t1, a0 -- 2.50.1