Re: [PATCH v2] usb: f_fs: Fix use-after-free for epfile
From: John Keeping <hidden>
Date: 2021-11-22 13:15:36
On Fri, Nov 19, 2021 at 03:38:49PM +0530, Udipto Goswami wrote:
Consider a case where ffs_func_eps_disable is called from
ffs_func_disable as part of composition switch and at the
same time ffs_epfile_release get called from userspace.
ffs_epfile_release will free up the read buffer and call
ffs_data_closed which in turn destroys ffs->epfiles and
mark it as NULL. While this was happening the driver has
already initialized the local epfile in ffs_func_eps_disable
which is now freed and waiting to acquire the spinlock. Once
spinlock is acquired the driver proceeds with the stale value
of epfile and tries to free the already freed read buffer
causing use-after-free.
Following is the illustration of the race:
CPU1 CPU2
ffs_func_eps_disable
epfiles (local copy)
ffs_epfile_release
__ffs_epfile_read_buffer_free
kfree(read_buffers)
kfree(epfile)
(epfiles still accessible
since local copy)
kfree(read_buffers) <use_after_free>
For added clarity here, it looks like the issue is more like:
CPU1 CPU2
ffs_func_eps_disable
epfiles (local copy)
ffs_epfile_release
ffs_data_closed
if (last file closed)
ffs_data_reset
ffs_data_clear
ffs_epfiles_destroy
free(epfiles)
spin_lock
dereference epfiles
leading to a use after free, is that correct?
In this case CPU1 may be called via set_alt() from the UDC code and CPU2
is from a userspace close().
Another possibility of user after free is with the read_buffers
Currently, ffs_func_eps_disable & ffs_epfile_release can race,
if ffs_epfile_release ran in between while ffs_func_eps_disable
was executing, due to not being in any lock it can go ahead
and free the read buffer, but since ffs_func_eps_disable
maintains a local copy of epfiles, it will still be valid here
which when tried to free again will cause a user_after_free.
Following is the illustration of the case:
CPU1 CPU2
ffs_func_eps_disable
spin_lock_irqsave
(epfile) local copy
ffs_epfile_release
__ffs_epfile_read_buffer_free
kfree(epfile->read_buffer)
__ffs_epfile_read_buffer_free
kfree(epfile->read_buffer)
<<use_after_free>>I don't see how this race can happen. __ffs_epfile_read_buffer_free() does: struct ffs_buffer *buf = xchg(&epfile->read_buffer, READ_BUFFER_DROP); if (buf && buf != READ_BUFFER_DROP) kfree(buf); so there's no way for both threads to call kfree() on the same buffer here.
quoted hunk ↗ jump to hunk
Fix this races by taking epfile local copy & assigning it under spinlock and if epfile(local) is null then update it in ffs->epfiles then finally destroy it. Signed-off-by: Pratham Pratap <redacted> Co-developed-by: Udipto Goswami <redacted> Signed-off-by: Udipto Goswami <redacted> --- v2: Fixed the tags. drivers/usb/gadget/function/f_fs.c | 44 +++++++++++++++++++++++++++++--------- 1 file changed, 34 insertions(+), 10 deletions(-)diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c index 3c584da..3cdc636 100644 --- a/drivers/usb/gadget/function/f_fs.c +++ b/drivers/usb/gadget/function/f_fs.c@@ -1268,10 +1268,13 @@ static int ffs_epfile_release(struct inode *inode, struct file *file) { struct ffs_epfile *epfile = inode->i_private; + unsigned long flags; ENTER(); + spin_lock_irqsave(&epfile->ffs->eps_lock, flags); __ffs_epfile_read_buffer_free(epfile); + spin_unlock_irqrestore(&epfile->ffs->eps_lock, flags);
Can you explain why the comment in struct ffs_epfile is wrong about the buffer synchronisation? At the very least, this patch looks like it should be updating that comment as well since the locking rules have clearly changed. But I'm not convinced this hunk is necessary - what is guarded here? epfile comes from the inode, so no need for eps_lock there and __ffs_epfile_read_buffer_free() accesses read_buffer via xchg() as described in ffs_epfile, so again no need for the lock.
quoted hunk ↗ jump to hunk
ffs_data_closed(epfile->ffs); return 0;@@ -1711,16 +1714,23 @@ static void ffs_data_put(struct ffs_data *ffs) static void ffs_data_closed(struct ffs_data *ffs) { + struct ffs_epfile *epfile;
Should this be epfiles to match the field it is saving?
quoted hunk ↗ jump to hunk
+ unsigned long flags; ENTER(); if (atomic_dec_and_test(&ffs->opened)) { if (ffs->no_disconnect) { ffs->state = FFS_DEACTIVATED; - if (ffs->epfiles) { - ffs_epfiles_destroy(ffs->epfiles, - ffs->eps_count); - ffs->epfiles = NULL; - } + spin_lock_irqsave(&ffs->eps_lock, flags); + epfile = ffs->epfiles; + ffs->epfiles = NULL; + spin_unlock_irqrestore(&ffs->eps_lock, + flags); + + if (epfile) + ffs_epfiles_destroy(epfile, + ffs->eps_count); + if (ffs->setup_state == FFS_SETUP_PENDING) __ffs_ep0_stall(ffs); } else {@@ -1767,14 +1777,25 @@ static struct ffs_data *ffs_data_new(const char *dev_name) static void ffs_data_clear(struct ffs_data *ffs) { + struct ffs_epfile *epfile;
Again, epfiles?
quoted hunk ↗ jump to hunk
+ unsigned long flags; ENTER(); ffs_closed(ffs); BUG_ON(ffs->gadget); - if (ffs->epfiles) - ffs_epfiles_destroy(ffs->epfiles, ffs->eps_count); + spin_lock_irqsave(&ffs->eps_lock, flags); + epfile = ffs->epfiles; + ffs->epfiles = NULL; + spin_unlock_irqrestore(&ffs->eps_lock, flags); + /* potential race possible between ffs_func_eps_disable + * & ffs_epfile_release therefore maintaining a local + * copy of epfile will save us from use-after-free. + */ + if (epfile) + ffs_epfiles_destroy(epfile, + ffs->eps_count); if (ffs->ffs_eventfd) eventfd_ctx_put(ffs->ffs_eventfd);@@ -1919,12 +1940,15 @@ static void ffs_epfiles_destroy(struct ffs_epfile *epfiles, unsigned count) static void ffs_func_eps_disable(struct ffs_function *func) { - struct ffs_ep *ep = func->eps; - struct ffs_epfile *epfile = func->ffs->epfiles; - unsigned count = func->ffs->eps_count; + struct ffs_ep *ep; + struct ffs_epfile *epfile;
epfiles?
+ unsigned short count;
Why change this to "unsigned short"?
unsigned long flags;
spin_lock_irqsave(&func->ffs->eps_lock, flags);
+ count = func->ffs->eps_count;
+ epfile = func->ffs->epfiles;
+ ep = func->eps;
while (count--) {
/* pending requests get nuked */
if (ep->ep)
--
2.7.4