Re: [PATCH 5.15.y] x86/mm/pat: fix VM_PAT handling in COW mappings

David Hildenbrand
2024-04-10T09:56:13Z

On 10.04.24 11:34, David Hildenbrand wrote:
> PAT handling won't do the right thing in COW mappings: the first PTE (or,
> in fact, all PTEs) can be replaced during write faults to point at anon
> folios. Reliably recovering the correct PFN and cachemode using
> follow_phys() from PTEs will not work in COW mappings.
>
> Using follow_phys(), we might just get the address+protection of the anon
> folio (which is very wrong), or fail on swap/nonswap entries, failing
> follow_phys() and triggering a WARN_ON_ONCE() in untrack_pfn() and
> track_pfn_copy(), not properly calling free_pfn_range().
>
> In free_pfn_range(), we either wouldn't call memtype_free() or would call
> it with the wrong range, possibly leaking memory.
>
> To fix that, let's update follow_phys() to refuse returning anon folios,
> and fallback to using the stored PFN inside vma->vm_pgoff for COW mappings
> if we run into that.
>
> We will now properly handle untrack_pfn() with COW mappings, where we
> don't need the cachemode. We'll have to fail fork()->track_pfn_copy() if
> the first page was replaced by an anon folio, though: we'd have to store
> the cachemode in the VMA to make this work, likely growing the VMA size.
>
> For now, lets keep it simple and let track_pfn_copy() just fail in that
> case: it would have failed in the past with swap/nonswap entries already,
> and it would have done the wrong thing with anon folios.
>
> Simple reproducer to trigger the WARN_ON_ONCE() in untrack_pfn():
>
> <--- C reproducer --->
> #include <stdio.h>
> #include <sys/mman.h>
> #include <unistd.h>
> #include <liburing.h>
>
> int main(void)
> {
> struct io_uring_params p = {};
> int ring_fd;
> size_t size;
> char *map;
>
> ring_fd = io_uring_setup(1, &p);
> if (ring_fd < 0) {
> perror("io_uring_setup");
> return 1;
> }
> size = p.sq_off.array + p.sq_entries * sizeof(unsigned);
>
> /* Map the submission queue ring MAP_PRIVATE */
> map = mmap(0, size, PROT_READ | PROT_WRITE, MAP_PRIVATE,
> ring_fd, IORING_OFF_SQ_RING);
> if (map == MAP_FAILED) {
> perror("mmap");
> return 1;
> }
>
> /* We have at least one page. Let's COW it. */
> *map = 0;
> pause();
> return 0;
> }
> <--- C reproducer --->
>
> On a system with 16 GiB RAM and swap configured:
> # ./iouring &
> # memhog 16G
> # killall iouring
> [ 301.552930] ------------[ cut here ]------------
> [ 301.553285] WARNING: CPU: 7 PID: 1402 at arch/x86/mm/pat/memtype.c:1060 untrack_pfn+0xf4/0x100
> [ 301.553989] Modules linked in: binfmt_misc nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_g
> [ 301.558232] CPU: 7 PID: 1402 Comm: iouring Not tainted 6.7.5-100.fc38.x86_64 #1
> [ 301.558772] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebu4
> [ 301.559569] RIP: 0010:untrack_pfn+0xf4/0x100
> [ 301.559893] Code: 75 c4 eb cf 48 8b 43 10 8b a8 e8 00 00 00 3b 6b 28 74 b8 48 8b 7b 30 e8 ea 1a f7 000
> [ 301.561189] RSP: 0018:ffffba2c0377fab8 EFLAGS: 00010282
> [ 301.561590] RAX: 00000000ffffffea RBX: ffff9208c8ce9cc0 RCX: 000000010455e047
> [ 301.562105] RDX: 07fffffff0eb1e0a RSI: 0000000000000000 RDI: ffff9208c391d200
> [ 301.562628] RBP: 0000000000000000 R08: ffffba2c0377fab8 R09: 0000000000000000
> [ 301.563145] R10: ffff9208d2292d50 R11: 0000000000000002 R12: 00007fea890e0000
> [ 301.563669] R13: 0000000000000000 R14: ffffba2c0377fc08 R15: 0000000000000000
> [ 301.564186] FS: 0000000000000000(0000) GS:ffff920c2fbc0000(0000) knlGS:0000000000000000
> [ 301.564773] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 301.565197] CR2: 00007fea88ee8a20 CR3: 00000001033a8000 CR4: 0000000000750ef0
> [ 301.565725] PKRU: 55555554
> [ 301.565944] Call Trace:
> [ 301.566148] <TASK>
> [ 301.566325] ? untrack_pfn+0xf4/0x100
> [ 301.566618] ? __warn+0x81/0x130
> [ 301.566876] ? untrack_pfn+0xf4/0x100
> [ 301.567163] ? report_bug+0x171/0x1a0
> [ 301.567466] ? handle_bug+0x3c/0x80
> [ 301.567743] ? exc_invalid_op+0x17/0x70
> [ 301.568038] ? asm_exc_invalid_op+0x1a/0x20
> [ 301.568363] ? untrack_pfn+0xf4/0x100
> [ 301.568660] ? untrack_pfn+0x65/0x100
> [ 301.568947] unmap_single_vma+0xa6/0xe0
> [ 301.569247] unmap_vmas+0xb5/0x190
> [ 301.569532] exit_mmap+0xec/0x340
> [ 301.569801] __mmput+0x3e/0x130
> [ 301.570051] do_exit+0x305/0xaf0
> ...
>
> Link: https://lkml.kernel.org/r/20240403212131.929421-3-david@redhat.com
> Signed-off-by: David Hildenbrand <david@redhat.com>
> Reported-by: Wupeng Ma <mawupeng1@huawei.com>
> Closes: https://lkml.kernel.org/r/20240227122814.3781907-1-mawupeng1@huawei.com
> Fixes: b1a86e15dc03 ("x86, pat: remove the dependency on 'vm_pgoff' in track/untrack pfn vma routines")
> Fixes: 5899329b1910 ("x86: PAT: implement track/untrack of pfnmap regions for x86 - v3")
> Acked-by: Ingo Molnar <mingo@kernel.org>
> Cc: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: "H. Peter Anvin" <hpa@zytor.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit 04c35ab3bdae7fefbd7c7a7355f29fa03a035221)
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---

Backport notes:
(1) vm_normal_folio -> vm_normal_page

--
Cheers,

David / dhildenb