Re: [PATCH v2] mm/secretmem: don't allow highmem folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 7/17/26 11:48, Brendan Jackman wrote:
> 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 1507f51255c9f ("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: https://github.com/bjackman/limmat-kernel-nix/commit/7b2acba2d3a5ef01400d493a155beb1d135b6bb5
> Suggested-by: Andrew Morton <akpm@linux-foundation.org>
> Link: https://lore.kernel.org/all/20260704192603.40aa80cf9242b77aa75e8d8d@linux-foundation.org/
> Fixes: 1507f51255c9f ("mm: introduce memfd_secret system call to create "secret" memory areas")
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
> Based on mm-new with `[PATCH] mm/secretmem: disable under HIGHMEM` [1]
> reverted.
> 
> Note this was found by Sashiko but in a local instance, so I can't
> provide a link to the report and I guess it's technically inaccurate to
> mention sashiko-bot@kernel.org.

We sometimes just used Reported-by: Sashiko ...without a link report.

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David