Hi Greg,
On 21-12-2021 01:16 pm, Greg Kroah-Hartman wrote:
On Mon, Dec 20, 2021 at 07:16:14PM +0530, Udipto Goswami wrote:
Hi Greg,
Apologies for the delay, please find the following comments as per my
understanding inline.
On 13-12-2021 07:38 pm, Greg Kroah-Hartman wrote:
On Tue, Dec 07, 2021 at 11:37:54AM +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_data_closed
if (last file closed)
ffs_data_reset
ffs_data_clear
ffs_epfiles_destroy
spin_lock
dereference epfiles
Fix this races by taking epfiles local copy & assigning it under
spinlock and if epfiles(local) is null then update it in ffs->epfiles
then finally destroy it.
Fixes: a9e6f83c2df (usb: gadget: f_fs: stop sleeping in
ffs_func_eps_disable)
Reviewed-by: John Keeping <john@xxxxxxxxxxxx>
Signed-off-by: Pratham Pratap <quic_ppratap@xxxxxxxxxxx>
Co-developed-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
Signed-off-by: Udipto Goswami <quic_ugoswami@xxxxxxxxxxx>
---
v6: Addressing minor code formatting.
drivers/usb/gadget/function/f_fs.c | 44 +++++++++++++++++++++++++++++---------
1 file changed, 34 insertions(+), 10 deletions(-)
What commit does this fix?
Does this need to go to stable kernel releases?
thanks,
greg k-h
This is fixing Commit: a9e6f83c2df (usb: gadget: f_fs: stop sleeping in
ffs_func_eps_disable)
Yes this fix is needed for >=5.4 Kernel releases.
Please put that in the patch when you resend it.
diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 3c584da..6f23a66 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -1711,16 +1711,24 @@ static void ffs_data_put(struct ffs_data *ffs)
static void ffs_data_closed(struct ffs_data *ffs)
{
+ struct ffs_epfile *epfiles;
+ 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);
+ epfiles = ffs->epfiles;
+ ffs->epfiles = NULL;
+ spin_unlock_irqrestore(&ffs->eps_lock,
+ flags);
+
+ if (epfiles)
+ ffs_epfiles_destroy(epfiles,
+ ffs->eps_count);
You are accessing epfiles outside of the lock. How is that ok?
There are 2 parties that are trying to free the ffs->epfiles. We use the
lock to serialize which party gets to free the ffs->epfiles. Where we are
making it NULL we protected that, The freeing part doesn't need to be inside
spinlock because now it is epfiles(local_ variable) with which we are trying
to operate now. We removed the ffs->epfiles as the common structure to
operate and made the local variable epfiles to do the job.
Why is the lock used only for that one field and not all other fields in
that structure? What about the no_disconnect access above these lines?
The setup_state access below them? Why does only one pointer deserve
this locking and not all of them?
I believe what you are trying to reference for protecting the ffs_data
structure is a little tricky. I understand that the entire ffs_data
isn't itself protected between concurrent cases but it will be probably
a bigger redesign which is out of scope of this patch.
Here we were trying only to protect the epfiles structure, which we get
from ffs_epfiles_create() ffs_func_eps_enable() is where we assign it
and ffs_data_reset() is where we are clearing it.
Maybe we can broaden the scope of this patch and try to protect the
epfiles in all the places we are using?
something like this?
@@ -1811,7 +1811,6 @@ static void ffs_data_reset(struct ffs_data *ffs)
ffs_data_clear(ffs);
- ffs->epfiles = NULL;
ffs->raw_descs_data = NULL;
ffs->raw_descs = NULL;
ffs->raw_strings = NULL;
@@ -1899,6 +1898,7 @@ static int ffs_epfiles_create(struct ffs_data *ffs)
if (!epfiles)
return -ENOMEM;
+ spin_lock_irqsave(&ffs->eps_lock, flags);
epfile = epfiles;
for (i = 1; i <= count; ++i, ++epfile) {
epfile->ffs = ffs;
@@ -1912,11 +1912,13 @@ static int ffs_epfiles_create(struct ffs_data
*ffs) &ffs_epfile_operations);
if (!epfile->dentry) {
ffs_epfiles_destroy(epfiles, i - 1);
+ spin_unlock_irqrestore(&ffs->eps_lock, flags);
should we be adding epfiles = NULL?
return -ENOMEM;
}
}
ffs->epfiles = epfiles;
+ spin_unlock_irqrestore(&ffs->eps_lock, flags);
return 0;
}
@@ -1966,14 +1968,18 @@ static void ffs_func_eps_disable(struct
ffs_function *func)
static int ffs_func_eps_enable(struct ffs_function *func)
{
- struct ffs_data *ffs = func->ffs;
- struct ffs_ep *ep = func->eps;
- struct ffs_epfile *epfile = ffs->epfiles;
- unsigned count = ffs->eps_count;
+ struct ffs_data *ffs;
+ struct ffs_ep *ep;
+ struct ffs_epfile *epfile;
+ unsigned count;
unsigned long flags;
int ret = 0;
spin_lock_irqsave(&func->ffs->eps_lock, flags);
+ ffs = func->ffs;
+ ep = func->eps;
+ epfiles = ffs->epfiles;
+ count = ffs->eps_count;
while(count--) {
ep->ep->driver_data = ep;
This is the intention actually to protect the epfiles. This additional
snippet will cover other EP related structures, and concurrent accesses
which i have mentioned in the illustration of the race.
We are more interested in protecting epfiles is because we have multiple
report from our customers indicating epfiles is accessed after free and
it is fixed with this patch.
We can work on the ffs_data protection seperately if required.
Thanks,
-Udipto
