On Wed, Dec 4, 2024 at 10:09 AM Kasireddy, Vivek <vivek.kasireddy@xxxxxxxxx> wrote: > > Subject: [PATCH 1/3] udmabuf: fix racy memfd sealing check > > > > The current check_memfd_seals() is racy: Since we first do > > check_memfd_seals() and then udmabuf_pin_folios() without holding any > > relevant lock across both, F_SEAL_WRITE can be set in between. > > This is problematic because we can end up holding pins to pages in a > > write-sealed memfd. > > > > Fix it using the inode lock, that's probably the easiest way. > > In the future, we might want to consider moving this logic into memfd, > > especially if anyone else wants to use memfd_pin_folios(). > > > > Reported-by: Julian Orth <ju.orth@xxxxxxxxx> > > Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219106 > > Closes: > > https://lore.kernel.org/r/CAG48ez0w8HrFEZtJkfmkVKFDhE5aP7nz=obrimeTg > > pD+StkV9w@xxxxxxxxxxxxxx > > Fixes: fbb0de795078 ("Add udmabuf misc device") > > Cc: stable@xxxxxxxxxxxxxxx > > Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> > > --- > > drivers/dma-buf/udmabuf.c | 9 +++++---- > > 1 file changed, 5 insertions(+), 4 deletions(-) > > > > diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c > > index > > 8ce1f074c2d32a0a9f59ff7184359e37d56548c6..662b9a26e06668bf59ab36d0 > > 7c0648c7b02ee5ae 100644 > > --- a/drivers/dma-buf/udmabuf.c > > +++ b/drivers/dma-buf/udmabuf.c > > @@ -436,14 +436,15 @@ static long udmabuf_create(struct miscdevice > > *device, > > goto err; > > } > > > > + inode_lock_shared(memfd->f_inode); > I think having inode_lock_shared(file_inode(memfd)) looks a bit more cleaner. Good idea, changed that. > Also, wouldn't it be more appropriate here to take the writer's lock instead > of the reader's lock given what we are doing (pinning) in udmabuf_create()? I don't see why that would require taking the inode lock in write mode. I am taking the inode lock to provide exclusion against memfd_add_seals(), which uses inode_lock(); in other words, the inode_lock is to protect the sealing status of the file from changing (which is a reader-like requirement). I'll add a comment in v2 to clarify this.
