Re: [PATCH] MIPS: Fix strnlen_user access check

[Tiezhu Yang]

On 04/11/2021 07:04 PM, Jinyang He wrote:
Commit 04324f44cb69 ("MIPS: Remove get_fs/set_fs") brought a problem for
strnlen_user(). Jump out when checking access_ok() with condition that
(s + strlen(s)) < __UA_LIMIT <= (s + n). The old __strnlen_user_asm()
just checked (ua_limit & s) without checking (ua_limit & (s + n)).
Therefore, find strlen form s to __UA_LIMIT - 1 in that condition.
Signed-off-by: Jinyang He <hejiny...@loongson.cn>
---
  arch/mips/include/asm/uaccess.h | 11 +++++++++--
  1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/mips/include/asm/uaccess.h b/arch/mips/include/asm/uaccess.h
index 91bc7fb..85ba0c8 100644
--- a/arch/mips/include/asm/uaccess.h
+++ b/arch/mips/include/asm/uaccess.h
@@ -630,8 +630,15 @@ static inline long strnlen_user(const char __user *s, long 
n)
  {
        long res;
  
-	if (!access_ok(s, n))
-               return -0;
+       if (unlikely(n <= 0))
+               return 0;
+
+       if (!access_ok(s, n)) {
+               if (!access_ok(s, 0))
+                       return 0;
+
+               n = __UA_LIMIT - (unsigned long)s - 1;
+       }
  
  	might_fault();
        __asm__ __volatile__(

The following simple changes are OK to fix this issue?

diff --git a/arch/mips/include/asm/uaccess.h b/arch/mips/include/asm/uaccess.h
index 91bc7fb..eafc99b 100644
--- a/arch/mips/include/asm/uaccess.h
+++ b/arch/mips/include/asm/uaccess.h
@@ -630,8 +630,8 @@ static inline long strnlen_user(const char __user *s, long 
n)
 {
        long res;
 
-       if (!access_ok(s, n))
-               return -0;
+       if (!access_ok(s, 1))
+               return 0;
 
        might_fault();
        __asm__ __volatile__(

Thanks,
Tiezhu