2020-06-01 20:09:04

by Eric Biggers

Subject: [PATCH v2] f2fs: avoid utf8_strncasecmp() with unstable name

From: Eric Biggers <[email protected]>

If the dentry name passed to ->d_compare() fits in dentry::d_iname, then
it may be concurrently modified by a rename. This can cause undefined
behavior (possibly out-of-bounds memory accesses or crashes) in
utf8_strncasecmp(), since fs/unicode/ isn't written to handle strings
that may be concurrently modified.

Fix this by first copying the filename to a stack buffer if needed.
This way we get a stable snapshot of the filename.

Fixes: 2c2eb7a300cd ("f2fs: Support case-insensitive file name lookups")
Cc: <[email protected]> # v5.4+
Cc: Al Viro <[email protected]>
Cc: Daniel Rosenberg <[email protected]>
Cc: Gabriel Krisman Bertazi <[email protected]>
Signed-off-by: Eric Biggers <[email protected]>
---

v2: Use memcpy() + barrier() instead of a byte-by-byte copy.
Also rebased onto f2fs/dev.

fs/f2fs/dir.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 29f70f2295cce..d35976785e8c5 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1114,11 +1114,27 @@ static int f2fs_d_compare(const struct dentry *dentry, unsigned int len,
const struct inode *dir = READ_ONCE(parent->d_inode);
const struct f2fs_sb_info *sbi = F2FS_SB(dentry->d_sb);
struct qstr entry = QSTR_INIT(str, len);
+ char strbuf[DNAME_INLINE_LEN];
int res;

if (!dir || !IS_CASEFOLDED(dir))
goto fallback;

+ /*
+ * If the dentry name is stored in-line, then it may be concurrently
+ * modified by a rename. If this happens, the VFS will eventually retry
+ * the lookup, so it doesn't matter what ->d_compare() returns.
+ * However, it's unsafe to call utf8_strncasecmp() with an unstable
+ * string. Therefore, we have to copy the name into a temporary buffer.
+ */
+ if (len <= DNAME_INLINE_LEN - 1) {
+ memcpy(strbuf, str, len);
+ strbuf[len] = 0;
+ entry.name = strbuf;
+ /* prevent compiler from optimizing out the temporary buffer */
+ barrier();
+ }
+
res = utf8_strncasecmp(sbi->s_encoding, name, &entry);
if (res >= 0)
return res;
--
2.26.2

2020-06-08 01:38:37

by Chao Yu

Subject: Re: [PATCH v2] f2fs: avoid utf8_strncasecmp() with unstable name

On 2020/6/2 4:08, Eric Biggers wrote:
> From: Eric Biggers <[email protected]>
>
> If the dentry name passed to ->d_compare() fits in dentry::d_iname, then
> it may be concurrently modified by a rename. This can cause undefined
> behavior (possibly out-of-bounds memory accesses or crashes) in
> utf8_strncasecmp(), since fs/unicode/ isn't written to handle strings
> that may be concurrently modified.
>
> Fix this by first copying the filename to a stack buffer if needed.
> This way we get a stable snapshot of the filename.
>
> Fixes: 2c2eb7a300cd ("f2fs: Support case-insensitive file name lookups")
> Cc: <[email protected]> # v5.4+
> Cc: Al Viro <[email protected]>
> Cc: Daniel Rosenberg <[email protected]>
> Cc: Gabriel Krisman Bertazi <[email protected]>
> Signed-off-by: Eric Biggers <[email protected]>

Reviewed-by: Chao Yu <[email protected]>

Thanks,