[PATCH] xattr: restrict vfs_getxattr_alloc() allocation size

[Jarkko Sakkinen]

On Thu Mar 7, 2024 at 10:01 PM EET, Jarkko Sakkinen wrote:
> On Tue Mar 5, 2024 at 2:27 PM EET, Christian Brauner wrote:
> > The vfs_getxattr_alloc() interface is a special-purpose in-kernel api
> > that does a racy query-size+allocate-buffer+retrieve-data. It is used by
> > EVM, IMA, and fscaps to retrieve xattrs. Recently, we've seen issues
> > where 9p returned values that amount to allocating about 8000GB worth of
> > memory (cf. [1]). That's now fixed in 9p. But vfs_getxattr_alloc() has
> > no reason to allow getting xattr values that are larger than
> > XATTR_MAX_SIZE as that's the limit we use for setting and getting xattr
> > values and nothing currently goes beyond that limit afaict. Let it check
> > for that and reject requests that are larger than that.
> >
> > Link: https://lore.kernel.org/r/ZeXcQmHWcYvfCR93@do-x1extreme [1]
> > Signed-off-by: Christian Brauner <brauner at kernel.org>
> > ---
> >  fs/xattr.c | 3 +++
> >  1 file changed, 3 insertions(+)
> >
> > diff --git a/fs/xattr.c b/fs/xattr.c
> > index 09d927603433..a53c930e3018 100644
> > --- a/fs/xattr.c
> > +++ b/fs/xattr.c
> > @@ -395,6 +395,9 @@ vfs_getxattr_alloc(struct mnt_idmap *idmap, struct dentry *dentry,
> >  	if (error < 0)
> >  		return error;
> >  
> > +	if (error > XATTR_SIZE_MAX)
> > +		return -E2BIG;
> > +
> >  	if (!value || (error > xattr_size)) {
> >  		value = krealloc(*xattr_value, error + 1, flags);
> >  		if (!value)
>
> I wonder if this should even categorized as a bug fix and get
> backported. Good catch!

Reviewed-by: Jarkko Sakkinen <jarkko at kernel.org>

BR, Jarkko