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From:   Matt Redfearn <matt.redfearn@mips.com>
To:     James Hogan <jhogan@kernel.org>, Ralf Baechle <ralf@linux-mips.org>
CC:     <linux-mips@linux-mips.org>,
        Matt Redfearn <matt.redfearn@mips.com>,
        <stable@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: [PATCH v2 1/4] MIPS: memset.S: Fix clobber of v1 in last_fixup
Date:   Tue, 17 Apr 2018 16:40:00 +0100
Message-ID: <1523979603-492-1-git-send-email-matt.redfearn@mips.com>
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-kernel-owner@vger.kernel.org
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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.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: stable@vger.kernel.org
Reported-by: James Hogan <jhogan@kernel.org>
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
---

Changes in v2: None

 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 184819c1d5c8..f7327979a8f8 100644
--- a/arch/mips/lib/memset.S
+++ b/arch/mips/lib/memset.S
@@ -258,7 +258,7 @@
 
 .Llast_fixup\@:
 	jr		ra
-	andi		v1, a2, STORMASK
+	 nop
 
 .Lsmall_fixup\@:
 	PTR_SUBU	a2, t1, a0
-- 
2.7.4