[v2,0/4] mm: clarify nofail memory allocation

Message

Barry Song July 31, 2024, 12:01 a.m. UTC

From: Barry Song <[e-mail]>


-v2:
 * adjust vpda fix according to Jason and Michal's feedback, I would
   expect Jason to test it, thanks!
 * split BUG_ON of unavoidable failure and the case GFP_ATOMIC |
   __GFP_NOFAIL into two patches according to Vlastimil and Michal.
 * collect Michal's acked-by for patch 2 - the doc;
 * remove the new GFP_NOFAIL from this series, that one would be a
   separate enhancement patchset later on.

-v1:
 https://lore.kernel.org/linux-mm/[e-mail]/

__GFP_NOFAIL carries the semantics of never failing, so its callers
do not check the return value:
  %__GFP_NOFAIL: The VM implementation _must_ retry infinitely: the caller
  cannot handle allocation failures. The allocation could block
  indefinitely but will never return with failure. Testing for
  failure is pointless.

However, __GFP_NOFAIL can sometimes fail if it exceeds size limits
or is used with GFP_ATOMIC/GFP_NOWAIT in a non-sleepable context.
This can expose security vulnerabilities due to potential NULL
dereferences.

Since __GFP_NOFAIL does not support non-blocking allocation, we introduce
GFP_NOFAIL with inclusive blocking semantics and encourage using GFP_NOFAIL
as a replacement for __GFP_NOFAIL in non-mm.

If we must still fail a nofail allocation, we should trigger a BUG rather
than exposing NULL dereferences to callers who do not check the return
value.

* The discussion started from this topic:
 [PATCH RFC] mm: warn potential return NULL for kmalloc_array and
             kvmalloc_array with __GFP_NOFAIL

 https://lore.kernel.org/linux-mm/[e-mail]/

Thank you to Michal, Christoph, Vlastimil, and Hailong for all the
comments.

Barry Song (4):
  vpda: try to fix the potential crash due to misusing __GFP_NOFAIL
  mm: Document __GFP_NOFAIL must be blockable
  mm: BUG_ON to avoid NULL deference while __GFP_NOFAIL fails
  mm: prohibit NULL deference exposed for unsupported non-blockable 
    __GFP_NOFAIL

 drivers/vdpa/vdpa_user/iova_domain.c | 31 +++++++++++++++++++++++-----
 drivers/vdpa/vdpa_user/iova_domain.h |  5 ++++-
 drivers/vdpa/vdpa_user/vduse_dev.c   |  4 +++-
 include/linux/gfp_types.h            |  5 ++++-
 include/linux/slab.h                 |  4 +++-
 mm/page_alloc.c                      | 14 +++++++------
 mm/util.c                            |  1 +
 7 files changed, 49 insertions(+), 15 deletions(-)