linux-hardening - [PATCH] fortify: strnlen: Call fortify_panic() only if the number of bytes read is greater than maxlen

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Message-ID: <20230724224857.2049906-1-sel4@tilde.club>
Date:   Mon, 24 Jul 2023 22:48:57 +0000
From:   sel4@...de.club
To:     kees@...nel.org
Cc:     keescook@...omium.org, Brandon Luo <sel4@...de.club>,
        syzbot+98d3ceb7e01269e7bf4f@...kaller.appspotmail.com,
        syzbot+155274e882dcbf9885df@...kaller.appspotmail.com,
        linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] fortify: strnlen: Call fortify_panic() only if the number of bytes read is greater than maxlen

From: Brandon Luo <sel4@...de.club>

If the number of bytes read is p_size and p_size is less than maxlen,
fortify_panic() will be called incorrectly. Only panic if the number of
bytes read is greater than the minimum of p_size and maxlen since that is
 the argument to __real_strnlen().

Reported-by: syzbot+98d3ceb7e01269e7bf4f@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000d8352e0600c0c804@google.com/

Reported-by: syzbot+155274e882dcbf9885df@...kaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000de4c2c0600c02b28@google.com/

Signed-off-by: Brandon Luo <sel4@...de.club>
---
 include/linux/fortify-string.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index da51a83b2829..cde637f735fe 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -176,8 +176,9 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const POS p, __kernel_size
 	}
 
 	/* Do not check characters beyond the end of p. */
-	ret = __real_strnlen(p, maxlen < p_size ? maxlen : p_size);
-	if (p_size <= ret && maxlen != ret)
+	maxlen = (maxlen < p_size) ? maxlen : p_size;
+	ret = __real_strnlen(p, maxlen);
+	if (maxlen < ret)
 		fortify_panic(__func__);
 	return ret;
 }
-- 
2.41.0