linux-kernel - [PATCH v2] maccess: fix strncpy_from_user

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Message-ID: <20250422131449.57177-1-mykyta.yatsenko5@gmail.com>
Date: Tue, 22 Apr 2025 14:14:49 +0100
From: Mykyta Yatsenko <mykyta.yatsenko5@...il.com>
To: akpm@...ux-foundation.org,
	linux-mm@...ck.org,
	rostedt@...dmis.org,
	mhiramat@...nel.org,
	andrii@...nel.org,
	kernel-team@...a.com,
	linux-kernel@...r.kernel.org
Cc: Mykyta Yatsenko <yatsenko@...a.com>
Subject: [PATCH v2] maccess: fix strncpy_from_user_nofault empty string handling

From: Mykyta Yatsenko <yatsenko@...a.com>

strncpy_from_user_nofault should return the length of the copied string
including the trailing NUL, but if the argument unsafe_addr points to
an empty string ({'\0'}), the return value is 0.

This happens as strncpy_from_user copies terminal symbol into dst
and returns 0 (as expected), but strncpy_from_user_nofault does not
modify ret as it is not equal to count and not greater than 0, so 0 is
returned, which contradicts the contract.

Signed-off-by: Mykyta Yatsenko <yatsenko@...a.com>
---
 mm/maccess.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/maccess.c b/mm/maccess.c
index 8f0906180a94..831b4dd7296c 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -196,7 +196,7 @@ long strncpy_from_user_nofault(char *dst, const void __user *unsafe_addr,
 	if (ret >= count) {
 		ret = count;
 		dst[ret - 1] = '\0';
-	} else if (ret > 0) {
+	} else if (ret >= 0) {
 		ret++;
 	}

-- 
2.49.0