Re: [PATCH] mm/secretmem: disable under HIGHMEM

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 03 Jul 2026 14:48:26 +0000 Brendan Jackman <jackmanb@google.com> wrote:

> secretmem_fault() allocates a folio with GFP_HIGHUSER and then calls
> set_direct_map_valid_noflush()

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,
> dropping support is not really a regression, so do that.

Well OK, but the secretmem code is still wrong.  The patch protects
people from hitting the bug but leaves the bug in place.  Surely it would be
better to fix the bug?

Is that as simple as adding the folio_test_highmem() test?  Or switching to
GFP_KERNEL?  You already have a reproducer (thanks), so this doesn't
sound like a lot of work?

(Should set_direct_map_invalid_noflush() WARN if passed a highmem page,
something like that?)