hugetlb does not support fake write-faults (write faults without write
permissions).  However, we are currently able to trigger a
FAULT_FLAG_WRITE fault on a VMA without VM_WRITE.

If we'd ever want to support FOLL_FORCE|FOLL_WRITE, we'd have to teach
hugetlb to:

(1) Leave the page mapped R/O after the fake write-fault, like
    maybe_mkwrite() does.
(2) Allow writing to an exclusive anon page that's mapped R/O when
    FOLL_FORCE is set, like can_follow_write_pte(). E.g.,
    __follow_hugetlb_must_fault() needs adjustment.

For now, it's not clear if that added complexity is really required. 
History tolds us that FOLL_FORCE is dangerous and that we better limit its
use to a bare minimum.

--------------------------------------------------------------------------
  #include <stdio.h>
  #include <stdlib.h>
  #include <fcntl.h>
  #include <unistd.h>
  #include <errno.h>
  #include <stdint.h>
  #include <sys/mman.h>
  #include <linux/mman.h>

  int main(int argc, char **argv)
  {
          char *map;
          int mem_fd;

          map = mmap(NULL, 2 * 1024 * 1024u, PROT_READ,
                     MAP_PRIVATE|MAP_ANON|MAP_HUGETLB|MAP_HUGE_2MB, -1, 0);
          if (map == MAP_FAILED) {
                  fprintf(stderr, "mmap() failed: %d\n", errno);
                  return 1;
          }

          mem_fd = open("/proc/self/mem", O_RDWR);
          if (mem_fd < 0) {
                  fprintf(stderr, "open(/proc/self/mem) failed: %d\n", errno);
                  return 1;
          }

          if (pwrite(mem_fd, "0", 1, (uintptr_t) map) == 1) {
                  fprintf(stderr, "write() succeeded, which is unexpected\n");
                  return 1;
          }

          printf("write() failed as expected: %d\n", errno);
          return 0;
  }
--------------------------------------------------------------------------

Fortunately, we have a sanity check in hugetlb_wp() in place ever since
commit 1d8d1464 ("mm/hugetlb: support write-faults in shared
mappings"), that bails out instead of silently mapping a page writable in
a !PROT_WRITE VMA.

Consequently, above reproducer triggers a warning, similar to the one
reported by szsbot:

------------[ cut here ]------------
WARNING: CPU: 1 PID: 3612 at mm/hugetlb.c:5313 hugetlb_wp+0x20a/0x1af0 mm/hugetlb.c:5313
Modules linked in:
CPU: 1 PID: 3612 Comm: syz-executor250 Not tainted 6.1.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
RIP: 0010:hugetlb_wp+0x20a/0x1af0 mm/hugetlb.c:5313
Code: ea 03 80 3c 02 00 0f 85 31 14 00 00 49 8b 5f 20 31 ff 48 89 dd 83 e5 02 48 89 ee e8 70 ab b7 ff 48 85 ed 75 5b e8 76 ae b7 ff <0f> 0b 41 bd 40 00 00 00 e8 69 ae b7 ff 48 b8 00 00 00 00 00 fc ff
RSP: 0018:ffffc90003caf620 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000008640070 RCX: 0000000000000000
RDX: ffff88807b963a80 RSI: ffffffff81c4ed2a RDI: 0000000000000007
RBP: 0000000000000000 R08: 0000000000000007 R09: 0000000000000000
R10: 0000000000000000 R11: 000000000008c07e R12: ffff888023805800
R13: 0000000000000000 R14: ffffffff91217f38 R15: ffff88801d4b0360
FS:  0000555555bba300(0000) GS:ffff8880b9b00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff7a47a1b8 CR3: 000000002378d000 CR4: 00000000003506e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 hugetlb_no_page mm/hugetlb.c:5755 [inline]
 hugetlb_fault+0x19cc/0x2060 mm/hugetlb.c:5874
 follow_hugetlb_page+0x3f3/0x1850 mm/hugetlb.c:6301
 __get_user_pages+0x2cb/0xf10 mm/gup.c:1202
 __get_user_pages_locked mm/gup.c:1434 [inline]
 __get_user_pages_remote+0x18f/0x830 mm/gup.c:2187
 get_user_pages_remote+0x84/0xc0 mm/gup.c:2260
 __access_remote_vm+0x287/0x6b0 mm/memory.c:5517
 ptrace_access_vm+0x181/0x1d0 kernel/ptrace.c:61
 generic_ptrace_pokedata kernel/ptrace.c:1323 [inline]
 ptrace_request+0xb46/0x10c0 kernel/ptrace.c:1046
 arch_ptrace+0x36/0x510 arch/x86/kernel/ptrace.c:828
 __do_sys_ptrace kernel/ptrace.c:1296 [inline]
 __se_sys_ptrace kernel/ptrace.c:1269 [inline]
 __x64_sys_ptrace+0x178/0x2a0 kernel/ptrace.c:1269
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]

So let's silence that warning by teaching GUP code that FOLL_FORCE -- so
far -- does not apply to hugetlb.

Note that FOLL_FORCE for read-access seems to be working as expected.  The
assumption is that this has been broken forever, only ever since above
commit, we actually detect the wrong handling and WARN_ON_ONCE().

I assume this has been broken at least since 2014, when mm/gup.c came to
life.  I failed to come up with a suitable Fixes tag quickly.

Link: https://lkml.kernel.org/r/20221031152524.173644-1-david@redhat.com
Fixes: 1d8d1464 ("mm/hugetlb: support write-faults in shared mappings")
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: <syzbot+f0b97304ef90f0d0b1dc@syzkaller.appspotmail.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>