mm/secretmem: don't allow highmem folios

secretmem_fault() allocates a folio with GFP_HIGHUSER and then calls
set_direct_map_invalid_noflush() without checking folio_test_highmem(). 
This causes a warning and process crash (vibe-coded reproducer in Link
below):

Su[   30.071284] ------------[ cut here ]------------
ccessfully allocated and mapped 2097152000 bytes at 0x3a449000
Populating memor[   30.074614] CPA: called for zero pte. vaddr = 0 cpa->vaddr = 0
y...
[   30.078636] WARNING: arch/x86/mm/pat/set_memory.c:1840 at __cpa_process_fault+0x34d/0x360, CPU#5: allocate_secret/570
[   30.084789] CPU: 5 UID: 0 PID: 570 Comm: allocate_secret Not tainted 7.1.0-14063-g4edcdefd4083-dirty #10 PREEMPTLAZY
[   30.090937] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.17.0-0-gb52ca86e094d-prebuilt.qemu.org 04/01/2014
[   30.097543] EIP: __cpa_process_fault+0x34d/0x360
[   30.100514] Code: ff ff 85 c0 0f 89 7d fe ff ff e9 3d fe ff ff 8b 03 8b 00 c7 04 24 c8 ff 64 c1 89 44 24 08 8b 45 e8 89 44 24 04 e8 53 7
a 00 00 <0f> 0b c7 45 f0 f2 ff ff ff e9 fc fc ff ff 90 8d 74 26 00 55 25 00
[   30.110829] EAX: 00000000 EBX: f64afe98 ECX: 00000000 EDX: 00000000
[   30.114799] ESI: 00000000 EDI: f64afe98 EBP: f64afe04 ESP: f64afdcc
[   30.118785] DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068 EFLAGS: 00010246
[   30.123020] CR0: 80050033 CR2: 46c48ffc CR3: 038c8000 CR4: 00000690
[   30.127010] Call Trace:
[   30.129078]  __change_page_attr_set_clr+0x5e7/0x870
[   30.132275]  ? console_unlock+0x99/0x130
[   30.135069]  ? irq_work_queue+0x36/0x70
[   30.137853]  ? page_address+0xd3/0xf0
[   30.140421]  set_direct_map_invalid_noflush+0x52/0x60
[   30.143782]  secretmem_fault+0x128/0x210
[   30.146560]  __do_fault+0x25/0x90
[   30.149053]  handle_mm_fault+0x6d1/0xcb0
[   30.151759]  exc_page_fault+0x135/0x3b0
[   30.154487]  ? doublefault_shim+0x150/0x150
[   30.157416]  handle_exception+0x130/0x130
[   30.160137] EIP: 0x804d29f
[   30.162307] Code: 89 54 08 e1 89 54 08 e5 89 54 08 e9 89 54 08 ed c3 0f b6 44 24 08 89 7c 24 0c 69 c0 01 01 01 01 8b 7c 24 04 f7 c7 0f 0
0 00 00 <89> 44 0f fc 75 0e c1 e9 02 f3 ab 8b 44 24 04 8b 7c 24 0c c3 31 d2
[   30.172936] EAX: 5a5a5a5a EBX: 00000000 ECX: 0c800000 EDX: 3a449000
[   30.176927] ESI: 00000000 EDI: 3a449000 EBP: bfbbae18 ESP: bfbbadac
[   30.180897] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00010246
[   30.185161]  ? doublefault_shim+0x150/0x150
[   30.187979] ---[ end trace 0000000000000000 ]---
Bus error                  (core dumped) ./allocate_secret_i686 2000M

The equivalent bug was pointed out by a local Sashiko instance on
https://lore.kernel.org/all/20260410151746.61150-3-kalyazin@amazon.com/

This hasn't been reproduced it on older kernel versions but from code
inspection the bug seems to go back to the original introduction in commit
1507f51255 ("mm: introduce memfd_secret system call to create "secret"
memory areas").  If this configuration has always been broken, there's no
need to worry too much about feature regression here.

Nonetheless, instead of just completely disabling secretmem under
!HIGHMEM, just drop __GFP_HIGHMEM.  This means that now where you
previously got a crash, instead you'll just see the secretmem process OOM.

Could secretmem just support highmem by saying "this isn't in the direct
map anyway" and bailing out before the set_direct_map_invalid_noflush()? 
Maybe.  That depends on requirements that are not well-defined (e.g.  is
it OK that kmap_local_page() is not a NOP for those pages?), and would
require some research and deep thinking.  Let's "defer" that until an
actual usecase arises.

Link: 7b2acba2d3
Link: https://lore.kernel.org/20260717-secretmem-highmem-v2-1-1f1a961ca91e@google.com
Link: https://lore.kernel.org/all/20260704192603.40aa80cf9242b77aa75e8d8d@linux-foundation.org/
Fixes: 1507f51255 ("mm: introduce memfd_secret system call to create "secret" memory areas")
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Brendan Jackman <brendan.jackman@linux.dev>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/secretmem.c

@@ -202,7 +202,7 @@ static struct file *secretmem_file_create(unsigned long flags)
 	if (IS_ERR(file))
 		goto err_free_inode;
 
-	mapping_set_gfp_mask(inode->i_mapping, GFP_HIGHUSER);
+	mapping_set_gfp_mask(inode->i_mapping, GFP_USER);
 	mapping_set_unevictable(inode->i_mapping);
 
 	inode->i_op = &secretmem_iops;