Re: [PATCH 5.4.y] x86/mm/pat: fix VM_PAT handling in COW mappings

[David Hildenbrand <david@redhat.com>]

On 10.04.24 11:49, 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) #include "../../mm/internal.h" to make it compile
(2) vm_normal_folio -> vm_normal_page
(3) follow_phys() is also used in generic_access_phys(), where we would
     now also reject anon pages. I don't think we care.


-- 
Cheers,

David / dhildenb