Al, any comments?
David's test-program is some broken mix of C and shell scripting, but
the fixed version does show the issue he talks about:
int main(int argc, char **argv)
{
int p[2], ro;
char buf[128];
pipe(p);
sprintf(buf, "/proc/self/fd/%d", p[1]);
ro = open(buf, O_RDONLY);
sprintf(buf, "/proc/self/fd/%d", ro);
close(p[1]);
return open(buf, O_RDWR);
}
which returns ETXTBSY (most easily seen by just stracing it).
The patch would also seem to make sense, with the i_readcount_inc()
being immediately below for the FMODE_READ case.
[ Quoting the whole email for context, sorry ]
Linus
On Mon, Mar 3, 2014 at 7:16 AM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> VM_DENYWRITE currently relies on i_writecount. Unless there's an active
> writable reference to an inode, VM_DENYWRITE is not allowed.
> Unfortunately, alloc_file() does not increase i_writecount, therefore,
> does not prevent a following VM_DENYWRITE even though the new file might
> have been opened with FMODE_WRITE. However, callers of alloc_file() expect
> the file object to be fully instantiated so they can call fput() on it. We
> could now either fix all callers to do an get_write_access() if opened
> with FMODE_WRITE, or simply fix alloc_file() to do that. I chose the
> latter.
>
> Note that this bug allows some rather subtle misbehavior. The following
> sequence of calls should work just fine, but currently fails:
> int p[2], orig, ro, rw;
> char buf[128];
>
> pipe(p);
> sprintf(buf, "/proc/self/fd/%d", p[1]);
> ro = open("/proc/self/fd/$orig", O_RDONLY);
> close(p[1]);
> rw = open("/proc/self/fd/$ro", O_RDWR);
>
> The final open() cannot succeed as close(p[1]) caused an integer underflow
> on i_writecount, effectively causing VM_DENYWRITE on the inode. The open
> will fail with -ETXTBUSY.
>
> It's a rather odd sequence of calls and given that open() doesn't use
> alloc_file() (and thus not affected by this bug), it's rather unlikely
> that this is a serious issue. But stuff like anon_inode shares a *single*
> inode across a huge set of interfaces. If any of these is broken like
> pipe(), it will affect all of these (ranging from dma-buf to epoll).
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: David Herrmann <dh.herrmann@xxxxxxxxx>
> ---
> fs/file_table.c | 27 ++++++++++++++++++---------
> 1 file changed, 18 insertions(+), 9 deletions(-)
>
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 5fff903..e3c8dd0 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -167,6 +167,7 @@ struct file *alloc_file(struct path *path, fmode_t mode,
> const struct file_operations *fop)
> {
> struct file *file;
> + int error;
>
> file = get_empty_filp();
> if (IS_ERR(file))
> @@ -178,15 +179,23 @@ struct file *alloc_file(struct path *path, fmode_t mode,
> file->f_mode = mode;
> file->f_op = fop;
>
> - /*
> - * These mounts don't really matter in practice
> - * for r/o bind mounts. They aren't userspace-
> - * visible. We do this for consistency, and so
> - * that we can do debugging checks at __fput()
> - */
> - if ((mode & FMODE_WRITE) && !special_file(path->dentry->d_inode->i_mode)) {
> - file_take_write(file);
> - WARN_ON(mnt_clone_write(path->mnt));
> + if (mode & FMODE_WRITE) {
> + error = get_write_access(path->dentry->d_inode);
> + if (error) {
> + put_filp(file);
> + return ERR_PTR(error);
> + }
> +
> + /*
> + * These mounts don't really matter in practice
> + * for r/o bind mounts. They aren't userspace-
> + * visible. We do this for consistency, and so
> + * that we can do debugging checks at __fput()
> + */
> + if (!special_file(path->dentry->d_inode->i_mode)) {
> + file_take_write(file);
> + WARN_ON(mnt_clone_write(path->mnt));
> + }
> }
> if ((mode & (FMODE_READ | FMODE_WRITE)) == FMODE_READ)
> i_readcount_inc(path->dentry->d_inode);
> --
> 1.9.0
>
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