Re: [PATCH v2 2/3] x86/mm/pat: fix VM_PAT handling in COW mappings

[mawupeng]

On 2024/4/4 5:21, 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
> ...
> 
> Reported-by: Wupeng Ma <mawupeng1@xxxxxxxxxx>
> Closes: https://lkml.kernel.org/r/20240227122814.3781907-1-mawupeng1@xxxxxxxxxx
> 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@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Borislav Petkov <bp@xxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>  arch/x86/mm/pat/memtype.c | 49 ++++++++++++++++++++++++++++-----------
>  mm/memory.c               |  4 ++++
>  2 files changed, 39 insertions(+), 14 deletions(-
Test-by: Wupeng Ma <mawupeng1@xxxxxxxxxx>

> 
> diff --git a/arch/x86/mm/pat/memtype.c b/arch/x86/mm/pat/memtype.c
> index 0d72183b5dd0..36b603d0cdde 100644
> --- a/arch/x86/mm/pat/memtype.c
> +++ b/arch/x86/mm/pat/memtype.c
> @@ -947,6 +947,38 @@ static void free_pfn_range(u64 paddr, unsigned long size)
>  		memtype_free(paddr, paddr + size);
>  }
>  
> +static int get_pat_info(struct vm_area_struct *vma, resource_size_t *paddr,
> +		pgprot_t *pgprot)
> +{
> +	unsigned long prot;
> +
> +	VM_WARN_ON_ONCE(!(vma->vm_flags & VM_PAT));
> +
> +	/*
> +	 * We need the starting PFN and cachemode used for track_pfn_remap()
> +	 * that covered the whole VMA. For most mappings, we can obtain that
> +	 * information from the page tables. For COW mappings, we might now
> +	 * suddenly have anon folios mapped and follow_phys() will fail.
> +	 *
> +	 * Fallback to using vma->vm_pgoff, see remap_pfn_range_notrack(), to
> +	 * detect the PFN. If we need the cachemode as well, we're out of luck
> +	 * for now and have to fail fork().
> +	 */
> +	if (!follow_phys(vma, vma->vm_start, 0, &prot, paddr)) {
> +		if (pgprot)
> +			*pgprot = __pgprot(prot);
> +		return 0;
> +	}
> +	if (is_cow_mapping(vma->vm_flags)) {
> +		if (pgprot)
> +			return -EINVAL;
> +		*paddr = (resource_size_t)vma->vm_pgoff << PAGE_SHIFT;
> +		return 0;
> +	}
> +	WARN_ON_ONCE(1);
> +	return -EINVAL;
> +}
> +
>  /*
>   * track_pfn_copy is called when vma that is covering the pfnmap gets
>   * copied through copy_page_range().
> @@ -957,20 +989,13 @@ static void free_pfn_range(u64 paddr, unsigned long size)
>  int track_pfn_copy(struct vm_area_struct *vma)
>  {
>  	resource_size_t paddr;
> -	unsigned long prot;
>  	unsigned long vma_size = vma->vm_end - vma->vm_start;
>  	pgprot_t pgprot;
>  
>  	if (vma->vm_flags & VM_PAT) {
> -		/*
> -		 * reserve the whole chunk covered by vma. We need the
> -		 * starting address and protection from pte.
> -		 */
> -		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
> -			WARN_ON_ONCE(1);
> +		if (get_pat_info(vma, &paddr, &pgprot))
>  			return -EINVAL;
> -		}
> -		pgprot = __pgprot(prot);
> +		/* reserve the whole chunk covered by vma. */
>  		return reserve_pfn_range(paddr, vma_size, &pgprot, 1);
>  	}
>  
> @@ -1045,7 +1070,6 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
>  		 unsigned long size, bool mm_wr_locked)
>  {
>  	resource_size_t paddr;
> -	unsigned long prot;
>  
>  	if (vma && !(vma->vm_flags & VM_PAT))
>  		return;
> @@ -1053,11 +1077,8 @@ void untrack_pfn(struct vm_area_struct *vma, unsigned long pfn,
>  	/* free the chunk starting from pfn or the whole chunk */
>  	paddr = (resource_size_t)pfn << PAGE_SHIFT;
>  	if (!paddr && !size) {
> -		if (follow_phys(vma, vma->vm_start, 0, &prot, &paddr)) {
> -			WARN_ON_ONCE(1);
> +		if (get_pat_info(vma, &paddr, NULL))
>  			return;
> -		}
> -
>  		size = vma->vm_end - vma->vm_start;
>  	}
>  	free_pfn_range(paddr, size);
> diff --git a/mm/memory.c b/mm/memory.c
> index 1211e2090c1a..1e9a0288fdaf 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6002,6 +6002,10 @@ int follow_phys(struct vm_area_struct *vma,
>  		goto out;
>  	pte = ptep_get(ptep);
>  
> +	/* Never return PFNs of anon folios in COW mappings. */
> +	if (vm_normal_folio(vma, address, pte))
> +		goto unlock;
> +
>  	if ((flags & FOLL_WRITE) && !pte_write(pte))
>  		goto unlock;
>