[PATCH AUTOSEL 6.19-5.15] fs/smb/client: fix out-of-bounds read in cifs_sanitize_prepath

[Sasha Levin <sashal@kernel.org>]

From: Fredric Cover <FredTheDude@proton.me>

[ Upstream commit 78ec5bf2f589ec7fd8f169394bfeca541b077317 ]

When cifs_sanitize_prepath is called with an empty string or a string
containing only delimiters (e.g., "/"), the current logic attempts to
check *(cursor2 - 1) before cursor2 has advanced. This results in an
out-of-bounds read.

This patch adds an early exit check after stripping prepended
delimiters. If no path content remains, the function returns NULL.

The bug was identified via manual audit and verified using a
standalone test case compiled with AddressSanitizer, which
triggered a SEGV on affected inputs.

Signed-off-by: Fredric Cover <FredTheDude@proton.me>
Reviewed-by: Henrique Carvalho <[2]henrique.carvalho@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

LLM Generated explanations, may be completely bogus:

The background agent confirmed my findings: the commit `78ec5bf2f589e`
was authored March 30, 2026, reviewed and committed by Steve French the
next day, merged via `v7.0-rc6-smb3-client-fix`. Lore was inaccessible
due to Anubis bot protection. No controversy found in the git record.

My analysis and decision stand as written above. This is a clear, small,
obviously correct fix for an out-of-bounds read in a widely-used
filesystem (SMB/CIFS), present since v5.16, with very low regression
risk.

**YES**

 fs/smb/client/fs_context.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/smb/client/fs_context.c b/fs/smb/client/fs_context.c
index e0d2cd78c82f1..e61bb6ac1d111 100644
--- a/fs/smb/client/fs_context.c
+++ b/fs/smb/client/fs_context.c
@@ -589,6 +589,10 @@ char *cifs_sanitize_prepath(char *prepath, gfp_t gfp)
 	while (IS_DELIM(*cursor1))
 		cursor1++;
 
+	/* exit in case of only delimiters */
+	if (!*cursor1)
+		return NULL;
+
 	/* copy the first letter */
 	*cursor2 = *cursor1;
 
-- 
2.53.0