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Subject[PATCH] fortify: strnlen: Call fortify_panic() only if the number of bytes read is greater than maxlen
Date
From: Brandon Luo <sel4@tilde.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@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000d8352e0600c0c804@google.com/

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

Signed-off-by: Brandon Luo <sel4@tilde.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
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