Commit c2661f5f authored Feb 09, 2025 by David Laight Committed by Andrew Morton Mar 16, 2025

mm: remove the access_ok() call from gup_fast_fallback()

Historiaclly the code relied on access_ok() to validate the address range.
Commit 26f4c328 added an explicit wrap check before access_ok().
Commit c28b1fc7 then changed the wrap test to use check_add_overflow().
Commit 6014bc27 relaxed the checks in x86-64's access_ok() and added
  an explicit check for TASK_SIZE here to make up for it.
That left a pointless access_ok() call with its associated 'lfence' that
can never actually fail.

So just delete the test.

Link: https://lkml.kernel.org/r/20250209174711.60889-1-david.laight.linux@gmail.com


Signed-off-by: David Laight <david.laight.linux@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jan Kara <jack@suse.cz>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

parent 67254c7d

mm/gup.c

+1 −3

Original line number	Diff line number	Diff line
		@@ -2760,7 +2760,7 @@ EXPORT_SYMBOL(get_user_pages_unlocked);
		*
		* *) ptes can be read atomically by the architecture.
		*
		* *) access_ok is sufficient to validate userspace address ranges.
		* *) valid user addesses are below TASK_MAX_SIZE
		*
		* The last two assumptions can be relaxed by the addition of helper functions.
		*
		@@ -3414,8 +3414,6 @@ static int gup_fast_fallback(unsigned long start, unsigned long nr_pages,
		return -EOVERFLOW;
		if (end > TASK_SIZE_MAX)
		return -EFAULT;
		if (unlikely(!access_ok((void __user *)start, len)))
		return -EFAULT;

		nr_pinned = gup_fast(start, end, gup_flags, pages);
		if (nr_pinned == nr_pages \|\| gup_flags & FOLL_FAST_ONLY)