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From:   Jinyang He <hejinyang@loongson.cn>
To:     Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc:     linux-mips@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH] MIPS: Fix strnlen_user access check
Date:   Sun, 11 Apr 2021 19:04:52 +0800
Message-Id: <1618139092-4018-1-git-send-email-hejinyang@loongson.cn>
Precedence: bulk

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 <hejinyang@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__(
-- 
2.1.0