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Subject[PATCH 1/6] fs: fix i_writecount on shmem and friends
Date
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(buf, O_RDONLY);
close(p[1]);
sprintf(buf, "/proc/self/fd/%d", ro);
rw = open(buf, 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).

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
Hi

This patch is only included for reference. It was submitted to fs-devel
separately and is being worked on. However, this bug must be fixed in order to
make use of memfd_create(), so I decided to include it here.

David

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 5b24008..8059d68 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -168,6 +168,7 @@ 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))
@@ -179,15 +180,23 @@ 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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