[v2] fs: Set file_handle::handle_bytes before referencing file_handle::f_handle

Commit Message

Kees Cook April 4, 2024, 9:12 p.m. UTC

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 <[e-mail]>
---
Cc: Jan Kara <[e-mail]>
Cc: Chuck Lever <[e-mail]>
Cc: Gustavo A. R. Silva <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Jeff Layton <[e-mail]>
Cc: Amir Goldstein <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
 v2: more bounds checking, add comments, dropped reviews since logic changed
 v1: https://lore.kernel.org/all/[e-mail]/
---
 fs/fhandle.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

Comments

Matthew Wilcox April 4, 2024, 9:29 p.m. UTC | #1

On Thu, Apr 04, 2024 at 02:12:15PM -0700, 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.

Why not change handle_bytes from an int to a u32?

Also, what a grotty function.

        handle_dwords = f_handle.handle_bytes >> 2;
...
        handle_bytes = handle_dwords * sizeof(u32);

> Fixes: 1b43c4629756 ("fs: Annotate struct file_handle with __counted_by() and use struct_size()")
> Signed-off-by: Kees Cook <[e-mail]>
> ---
> Cc: Jan Kara <[e-mail]>
> Cc: Chuck Lever <[e-mail]>
> Cc: Gustavo A. R. Silva <[e-mail]>
> Cc: Christian Brauner <[e-mail]>
> Cc: Alexander Viro <[e-mail]>
> Cc: Jeff Layton <[e-mail]>
> Cc: Amir Goldstein <[e-mail]>
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
>  v2: more bounds checking, add comments, dropped reviews since logic changed
>  v1: https://lore.kernel.org/all/[e-mail]/
> ---
>  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
> 
>

Jan Kara April 5, 2024, 10:21 a.m. UTC | #2

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 <[e-mail]>
> ---
> Cc: Jan Kara <[e-mail]>
> Cc: Chuck Lever <[e-mail]>
> Cc: Gustavo A. R. Silva <[e-mail]>
> Cc: Christian Brauner <[e-mail]>
> Cc: Alexander Viro <[e-mail]>
> Cc: Jeff Layton <[e-mail]>
> Cc: Amir Goldstein <[e-mail]>
> Cc: [e-mail]
> Cc: [e-mail]
> Cc: [e-mail]
>  v2: more bounds checking, add comments, dropped reviews since logic changed
>  v1: https://lore.kernel.org/all/[e-mail]/
> ---
>  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
>

Kees Cook April 5, 2024, 3:36 p.m. UTC | #3

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 <[e-mail]>
> > ---
> > Cc: Jan Kara <[e-mail]>
> > Cc: Chuck Lever <[e-mail]>
> > Cc: Gustavo A. R. Silva <[e-mail]>
> > Cc: Christian Brauner <[e-mail]>
> > Cc: Alexander Viro <[e-mail]>
> > Cc: Jeff Layton <[e-mail]>
> > Cc: Amir Goldstein <[e-mail]>
> > Cc: [e-mail]
> > Cc: [e-mail]
> > Cc: [e-mail]
> >  v2: more bounds checking, add comments, dropped reviews since logic changed
> >  v1: https://lore.kernel.org/all/[e-mail]/
> > ---
> >  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.

Patch

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,