Re: [PATCH v2] fs/netfs3: add a boundary check for EA_FULL

Dan Carpenter
2022-09-14T15:34:49Z

On Mon, Sep 12, 2022 at 02:54:31PM +0800, eadavis@sina.com wrote:
> From: Edward Adam Davis <eadavis@sina.com>
>
> the root cause is:
> The remaining space after the offset is less than the space needed to
> accommodate the next EA_FULL struct.
>

This needs to be checked on the first iteration as well before calling
unpacked_ea_size(ea).

if (bytes - *off < sizeof(*ea))
return false;

> Link: https://syzkaller.appspot.com/bug?extid=c4d950787fd5553287b7
> Reported-by: syzbot+c4d950787fd5553287b7@syzkaller.appspotmail.com
> Signed-off-by: Edward Adam Davis <eadavis@sina.com>
> ---
> fs/ntfs3/xattr.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c
> index 7de8718c68a9..c90cc453390d 100644
> --- a/fs/ntfs3/xattr.c
> +++ b/fs/ntfs3/xattr.c
> @@ -52,6 +52,7 @@ static inline bool find_ea(const struct EA_FULL *ea_all, u32 bytes,
> for (;;) {
> const struct EA_FULL *ea = Add2Ptr(ea_all, *off);
> u32 next_off = *off + unpacked_ea_size(ea);
> + u32 next_len = 0;
>
> if (next_off > bytes)
> return false;
> @@ -63,6 +64,13 @@ static inline bool find_ea(const struct EA_FULL *ea_all, u32 bytes,
> *off = next_off;
> if (next_off >= bytes)
> return false;
> +
> + next_len = next_off + 8;

8 is a magic number. Use sizeof(*ea).

> + if (next_len >= bytes ||
> + ((!ea->size) &&
> + (next_len + ea->name_len +
> + le16_to_cpu(ea->elength) >= bytes)))

This is open coding unpacked_ea_size() but slightly different/incorrect.
No need to check this anyway, because it gets checked at the start of
the iteration.

If we add the if (bytes - *off < sizeof(*ea)) check to the start of the
iteration that means we can delete the "if (next_off == bytes)" check
from the end.

The second issue with this code is that unpacked_ea_size() is a user
controlled u32. We're adding it to "*off" which is also a u32 so that
can have an integer overflow...

See diff below. (Untested).

regards,
dan carpenter

diff --git a/fs/ntfs3/xattr.c b/fs/ntfs3/xattr.c
index 7de8718c68a9..c3dbe06fb784 100644
--- a/fs/ntfs3/xattr.c
+++ b/fs/ntfs3/xattr.c
@@ -44,14 +44,20 @@ static inline size_t packed_ea_size(const struct EA_FULL *ea)
static inline bool find_ea(const struct EA_FULL *ea_all, u32 bytes,
const char *name, u8 name_len, u32 *off)
{
+ const struct EA_FULL *ea;
+ u32 next_off;
+
*off = 0;

- if (!ea_all || !bytes)
+ if (!ea_all)
return false;

for (;;) {
- const struct EA_FULL *ea = Add2Ptr(ea_all, *off);
- u32 next_off = *off + unpacked_ea_size(ea);
+ if (bytes - *off < sizeof(*ea))
+ return false;
+
+ ea = Add2Ptr(ea_all, *off);
+ next_off = size_add(*off, unpacked_ea_size(ea));

if (next_off > bytes)
return false;
@@ -61,8 +67,6 @@ static inline bool find_ea(const struct EA_FULL *ea_all, u32 bytes,
return true;

*off = next_off;
- if (next_off >= bytes)
- return false;
}
}