+ mm-secretmem-avoid-letting-secretmem_users-drop-to-zero.patch added to -mm tree

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The patch titled
     Subject: mm/secretmem: avoid letting secretmem_users drop to zero
has been added to the -mm tree.  Its filename is
     mm-secretmem-avoid-letting-secretmem_users-drop-to-zero.patch

This patch should soon appear at
    https://ozlabs.org/~akpm/mmots/broken-out/mm-secretmem-avoid-letting-secretmem_users-drop-to-zero.patch
and later at
    https://ozlabs.org/~akpm/mmotm/broken-out/mm-secretmem-avoid-letting-secretmem_users-drop-to-zero.patch

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------------------------------------------------------
From: Kees Cook <keescook@xxxxxxxxxxxx>
Subject: mm/secretmem: avoid letting secretmem_users drop to zero

Quoting Dmitry: "refcount_inc() needs to be done before fd_install(). 
After fd_install() finishes, the fd can be used by userspace and we can
have secret data in memory before the refcount_inc().

A straightforward misuse where a user will predict the returned fd in
another thread before the syscall returns and will use it to store secret
data is somewhat dubious because such a user just shoots themself in the
foot.

But a more interesting misuse would be to close the predicted fd and
decrement the refcount before the corresponding refcount_inc, this way one
can briefly drop the refcount to zero while there are other users of
secretmem."

Move fd_install() after refcount_inc().

Link: https://lkml.kernel.org/r/20211021154046.880251-1-keescook@xxxxxxxxxxxx
Link: https://lore.kernel.org/lkml/CACT4Y+b1sW6-Hkn8HQYw_SsT7X3tp-CJNh2ci0wG3ZnQz9jjig@xxxxxxxxxxxxxx
Fixes: 9a436f8ff631 ("PM: hibernate: disable when there are active secretmem users")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Reviewed-by: Jordy Zomer <jordy@pwning.systems>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/secretmem.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/secretmem.c~mm-secretmem-avoid-letting-secretmem_users-drop-to-zero
+++ a/mm/secretmem.c
@@ -217,8 +217,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned i
 
 	file->f_flags |= O_LARGEFILE;
 
-	fd_install(fd, file);
 	refcount_inc(&secretmem_users);
+	fd_install(fd, file);
 	return fd;
 
 err_put_fd:
_

Patches currently in -mm which might be from keescook@xxxxxxxxxxxx are

mm-secretmem-avoid-letting-secretmem_users-drop-to-zero.patch
kasan-test-bypass-__alloc_size-checks.patch
rapidio-avoid-bogus-__alloc_size-warning.patch
compiler-attributes-add-__alloc_size-for-better-bounds-checking.patch
slab-clean-up-function-prototypes.patch
slab-add-__alloc_size-attributes-for-better-bounds-checking.patch
mm-kvmalloc-add-__alloc_size-attributes-for-better-bounds-checking.patch
mm-vmalloc-add-__alloc_size-attributes-for-better-bounds-checking.patch
mm-page_alloc-add-__alloc_size-attributes-for-better-bounds-checking.patch
percpu-add-__alloc_size-attributes-for-better-bounds-checking.patch
kasan-test-consolidate-workarounds-for-unwanted-__alloc_size-protection.patch
maintainers-add-exec-binfmt-section-with-myself-and-eric.patch
binfmt_elf-reintroduce-using-map_fixed_noreplace.patch




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