Since __counted_by(handle_bytes) was added to struct file_handle, we need
to explicitly set it in the one place it wasn't yet happening prior to
accessing the flex array "f_handle". For robustness also check for a
negative value for handle_bytes, which is possible for an "int", but
nothing appears to set.
Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
Signed-off-by: Kees Cook <[email protected]>
---
Cc: Jan Kara <[email protected]>
Cc: Chuck Lever <[email protected]>
Cc: Gustavo A. R. Silva <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Alexander Viro <[email protected]>
Cc: Jeff Layton <[email protected]>
Cc: Amir Goldstein <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
v2: more bounds checking, add comments, dropped reviews since logic changed
v1: https://lore.kernel.org/all/[email protected]/
---
fs/fhandle.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/fs/fhandle.c b/fs/fhandle.c
index 8a7f86c2139a..854f866eaad2 100644
--- a/fs/fhandle.c
+++ b/fs/fhandle.c
@@ -40,6 +40,11 @@ static long do_sys_name_to_handle(const struct path *path,
GFP_KERNEL);
if (!handle)
return -ENOMEM;
+ /*
+ * Since handle->f_handle is about to be written, make sure the
+ * associated __counted_by(handle_bytes) variable is correct.
+ */
+ handle->handle_bytes = f_handle.handle_bytes;
/* convert handle size to multiple of sizeof(u32) */
handle_dwords = f_handle.handle_bytes >> 2;
@@ -51,8 +56,8 @@ static long do_sys_name_to_handle(const struct path *path,
handle->handle_type = retval;
/* convert handle size to bytes */
handle_bytes = handle_dwords * sizeof(u32);
- handle->handle_bytes = handle_bytes;
- if ((handle->handle_bytes > f_handle.handle_bytes) ||
+ /* check if handle_bytes would have exceeded the allocation */
+ if ((handle_bytes < 0) || (handle_bytes > f_handle.handle_bytes) ||
(retval == FILEID_INVALID) || (retval < 0)) {
/* As per old exportfs_encode_fh documentation
* we could return ENOSPC to indicate overflow
@@ -68,6 +73,8 @@ static long do_sys_name_to_handle(const struct path *path,
handle_bytes = 0;
} else
retval = 0;
+ /* the "valid" number of bytes may fewer than originally allocated */
+ handle->handle_bytes = handle_bytes;
/* copy the mount id */
if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
copy_to_user(ufh, handle,
--
2.34.1
On Thu 04-04-24 14:12:15, Kees Cook wrote:
> Since __counted_by(handle_bytes) was added to struct file_handle, we need
> to explicitly set it in the one place it wasn't yet happening prior to
> accessing the flex array "f_handle". For robustness also check for a
> negative value for handle_bytes, which is possible for an "int", but
> nothing appears to set.
>
> Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
> Signed-off-by: Kees Cook <[email protected]>
> ---
> Cc: Jan Kara <[email protected]>
> Cc: Chuck Lever <[email protected]>
> Cc: Gustavo A. R. Silva <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Alexander Viro <[email protected]>
> Cc: Jeff Layton <[email protected]>
> Cc: Amir Goldstein <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> v2: more bounds checking, add comments, dropped reviews since logic changed
> v1: https://lore.kernel.org/all/[email protected]/
> ---
> fs/fhandle.c | 11 +++++++++--
> 1 file changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/fs/fhandle.c b/fs/fhandle.c
> index 8a7f86c2139a..854f866eaad2 100644
> --- a/fs/fhandle.c
> +++ b/fs/fhandle.c
> @@ -40,6 +40,11 @@ static long do_sys_name_to_handle(const struct path *path,
> GFP_KERNEL);
> if (!handle)
> return -ENOMEM;
> + /*
> + * Since handle->f_handle is about to be written, make sure the
> + * associated __counted_by(handle_bytes) variable is correct.
> + */
> + handle->handle_bytes = f_handle.handle_bytes;
>
> /* convert handle size to multiple of sizeof(u32) */
> handle_dwords = f_handle.handle_bytes >> 2;
> @@ -51,8 +56,8 @@ static long do_sys_name_to_handle(const struct path *path,
> handle->handle_type = retval;
> /* convert handle size to bytes */
> handle_bytes = handle_dwords * sizeof(u32);
> - handle->handle_bytes = handle_bytes;
> - if ((handle->handle_bytes > f_handle.handle_bytes) ||
> + /* check if handle_bytes would have exceeded the allocation */
> + if ((handle_bytes < 0) || (handle_bytes > f_handle.handle_bytes) ||
This is broken. Let me explain: Userspace passes in struct file_handle
(ufh) and says how many bytes it has reserved for the variable length
contents in ufh->handle_bytes. We call exportfs_encode_fh() to create
the file handle. If it fits into the provided space, the function returns
in handle_dwords how many uints it has actually stored. If the handle
didn't fit, handle_dword contains number of uints we'd need in the variable
length part to be able to fit the handle in.
Now your patch destroys this behavior by storing 0 to handle_bytes in case
the handle didn't fit *before* the returned value is actually stored to a
struct copied to userspace.
Also the handle_bytes < 0 check is IMHO pointless and confusing. If some
filesystem is returning bogus size of the file handle, we have a big
problem anyway so I don't think checking like this brings much. If you want
to add some paranoia what would make some sense is: Make handle_bytes uint
and do:
if (WARN_ON_ONCE(handle_bytes > MAX_HANDLE_SZ)) {
handle_bytes = 0;
retval = -EINVAL;
}
But as a separate patch please, because it is unrelated to this __counted_by
fixup.
Honza
> (retval == FILEID_INVALID) || (retval < 0)) {
> /* As per old exportfs_encode_fh documentation
> * we could return ENOSPC to indicate overflow
> @@ -68,6 +73,8 @@ static long do_sys_name_to_handle(const struct path *path,
> handle_bytes = 0;
> } else
> retval = 0;
> + /* the "valid" number of bytes may fewer than originally allocated */
> + handle->handle_bytes = handle_bytes;
> /* copy the mount id */
> if (put_user(real_mount(path->mnt)->mnt_id, mnt_id) ||
> copy_to_user(ufh, handle,
> --
> 2.34.1
>
--
Jan Kara <[email protected]>
SUSE Labs, CR
On Fri, Apr 05, 2024 at 12:21:57PM +0200, Jan Kara wrote:
> On Thu 04-04-24 14:12:15, Kees Cook wrote:
> > Since __counted_by(handle_bytes) was added to struct file_handle, we need
> > to explicitly set it in the one place it wasn't yet happening prior to
> > accessing the flex array "f_handle". For robustness also check for a
> > negative value for handle_bytes, which is possible for an "int", but
> > nothing appears to set.
> >
> > Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
> > Signed-off-by: Kees Cook <[email protected]>
> > ---
> > Cc: Jan Kara <[email protected]>
> > Cc: Chuck Lever <[email protected]>
> > Cc: Gustavo A. R. Silva <[email protected]>
> > Cc: Christian Brauner <[email protected]>
> > Cc: Alexander Viro <[email protected]>
> > Cc: Jeff Layton <[email protected]>
> > Cc: Amir Goldstein <[email protected]>
> > Cc: [email protected]
> > Cc: [email protected]
> > Cc: [email protected]
> > v2: more bounds checking, add comments, dropped reviews since logic changed
> > v1: https://lore.kernel.org/all/[email protected]/
> > ---
> > fs/fhandle.c | 11 +++++++++--
> > 1 file changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/fhandle.c b/fs/fhandle.c
> > index 8a7f86c2139a..854f866eaad2 100644
> > --- a/fs/fhandle.c
> > +++ b/fs/fhandle.c
> > @@ -40,6 +40,11 @@ static long do_sys_name_to_handle(const struct path *path,
> > GFP_KERNEL);
> > if (!handle)
> > return -ENOMEM;
> > + /*
> > + * Since handle->f_handle is about to be written, make sure the
> > + * associated __counted_by(handle_bytes) variable is correct.
> > + */
> > + handle->handle_bytes = f_handle.handle_bytes;
> >
> > /* convert handle size to multiple of sizeof(u32) */
> > handle_dwords = f_handle.handle_bytes >> 2;
> > @@ -51,8 +56,8 @@ static long do_sys_name_to_handle(const struct path *path,
> > handle->handle_type = retval;
> > /* convert handle size to bytes */
> > handle_bytes = handle_dwords * sizeof(u32);
> > - handle->handle_bytes = handle_bytes;
> > - if ((handle->handle_bytes > f_handle.handle_bytes) ||
> > + /* check if handle_bytes would have exceeded the allocation */
> > + if ((handle_bytes < 0) || (handle_bytes > f_handle.handle_bytes) ||
>
> This is broken. Let me explain: Userspace passes in struct file_handle
> (ufh) and says how many bytes it has reserved for the variable length
> contents in ufh->handle_bytes. We call exportfs_encode_fh() to create
> the file handle. If it fits into the provided space, the function returns
> in handle_dwords how many uints it has actually stored. If the handle
> didn't fit, handle_dword contains number of uints we'd need in the variable
> length part to be able to fit the handle in.
>
> Now your patch destroys this behavior by storing 0 to handle_bytes in case
> the handle didn't fit *before* the returned value is actually stored to a
> struct copied to userspace.
Ah yes, I see now how they're used for separate things. I will respin
again and possibly fix up the "int" to "u32" as suggested by willy.
--
Kees Cook