[PATCH AUTOSEL 6.18] fs/smb/client: fix out-of-bounds read in cifs_sanitize_prepath

From: Fredric Cover <[email protected]>

[ 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 <[email protected]>
Reviewed-by: Henrique Carvalho <[2][email protected]>
Signed-off-by: Steve French <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
---

LLM Generated explanations, may be completely bogus:

Error: Failed to generate final synthesis

 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 be82acacc41d6..f207c7cef0467 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