This is a note to let you know that I've just added the patch titled mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. >From 73f37dbcfe1763ee2294c7717a1f571e27d17fd8 Mon Sep 17 00:00:00 2001 From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> Date: Fri, 10 Jun 2022 10:38:12 -0700 Subject: mm: userfaultfd: fix UFFDIO_CONTINUE on fallocated shmem pages From: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> commit 73f37dbcfe1763ee2294c7717a1f571e27d17fd8 upstream. When fallocate() is used on a shmem file, the pages we allocate can end up with !PageUptodate. Since UFFDIO_CONTINUE tries to find the existing page the user wants to map with SGP_READ, we would fail to find such a page, since shmem_getpage_gfp returns with a "NULL" pagep for SGP_READ if it discovers !PageUptodate. As a result, UFFDIO_CONTINUE returns -EFAULT, as it would do if the page wasn't found in the page cache at all. This isn't the intended behavior. UFFDIO_CONTINUE is just trying to find if a page exists, and doesn't care whether it still needs to be cleared or not. So, instead of SGP_READ, pass in SGP_NOALLOC. This is the same, except for one critical difference: in the !PageUptodate case, SGP_NOALLOC will clear the page and then return it. With this change, UFFDIO_CONTINUE works properly (succeeds) on a shmem file which has been fallocated, but otherwise not modified. Link: https://lkml.kernel.org/r/20220610173812.1768919-1-axelrasmussen@xxxxxxxxxx Fixes: 153132571f02 ("userfaultfd/shmem: support UFFDIO_CONTINUE for shmem") Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> Acked-by: Peter Xu <peterx@xxxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/userfaultfd.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -227,7 +227,10 @@ static int mcontinue_atomic_pte(struct m struct page *page; int ret; - ret = shmem_getpage(inode, pgoff, &page, SGP_READ); + ret = shmem_getpage(inode, pgoff, &page, SGP_NOALLOC); + /* Our caller expects us to return -EFAULT if we failed to find page. */ + if (ret == -ENOENT) + ret = -EFAULT; if (ret) goto out; if (!page) { Patches currently in stable-queue which might be from axelrasmussen@xxxxxxxxxx are queue-5.15/mm-userfaultfd-fix-uffdio_continue-on-fallocated-shmem-pages.patch