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

Historiaclly the code relied on access_ok() to validate the address range.
Commit 26f4c328079d7 added an explicit wrap check before access_ok().
Commit c28b1fc70390d then changed the wrap test to use check_add_overflow().
Commit 6014bc27561f2 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>

diff --git a/mm/gup.c b/mm/gup.c
index 61e751baf862c545320358d91f73818867880f26..e42e4fdaf7651e242b572017d38cfe753d880117 100644 (file)
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -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)