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Date:   Thu, 23 Mar 2023 10:52:35 +0100
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Subject: Re: [v4 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration
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To:     Yang Shi <shy828301@gmail.com>, kirill.shutemov@linux.intel.com,
        jannh@google.com, willy@infradead.org, david@redhat.com,
        akpm@linux-foundation.org
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
References: <20220203182641.824731-1-shy828301@gmail.com>
From:   Vlastimil Babka <vbabka@suse.cz>
In-Reply-To: <20220203182641.824731-1-shy828301@gmail.com>
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On 2/3/22 19:26, Yang Shi wrote:
> The syzbot reported the below BUG:
> 
> kernel BUG at include/linux/page-flags.h:785!
> invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> CPU: 1 PID: 4392 Comm: syz-executor560 Not tainted 5.16.0-rc6-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>  <TASK>
>  page_mapcount include/linux/mm.h:837 [inline]
>  smaps_account+0x470/0xb10 fs/proc/task_mmu.c:466
>  smaps_pte_entry fs/proc/task_mmu.c:538 [inline]
>  smaps_pte_range+0x611/0x1250 fs/proc/task_mmu.c:601
>  walk_pmd_range mm/pagewalk.c:128 [inline]
>  walk_pud_range mm/pagewalk.c:205 [inline]
>  walk_p4d_range mm/pagewalk.c:240 [inline]
>  walk_pgd_range mm/pagewalk.c:277 [inline]
>  __walk_page_range+0xe23/0x1ea0 mm/pagewalk.c:379
>  walk_page_vma+0x277/0x350 mm/pagewalk.c:530
>  smap_gather_stats.part.0+0x148/0x260 fs/proc/task_mmu.c:768
>  smap_gather_stats fs/proc/task_mmu.c:741 [inline]
>  show_smap+0xc6/0x440 fs/proc/task_mmu.c:822
>  seq_read_iter+0xbb0/0x1240 fs/seq_file.c:272
>  seq_read+0x3e0/0x5b0 fs/seq_file.c:162
>  vfs_read+0x1b5/0x600 fs/read_write.c:479
>  ksys_read+0x12d/0x250 fs/read_write.c:619
>  do_syscall_x64 arch/x86/entry/common.c:50 [inline]
>  do_syscall_64+0x35/0xb0 arch/x86/entry/common.c:80
>  entry_SYSCALL_64_after_hwframe+0x44/0xae
> RIP: 0033:0x7faa2af6c969
> Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 11 15 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b8 ff ff ff f7 d8 64 89 01 48
> RSP: 002b:00007faa2aefd288 EFLAGS: 00000246 ORIG_RAX: 0000000000000000
> RAX: ffffffffffffffda RBX: 00007faa2aff4418 RCX: 00007faa2af6c969
> RDX: 0000000000002025 RSI: 0000000020000100 RDI: 0000000000000003
> RBP: 00007faa2aff4410 R08: 00007faa2aefd700 R09: 0000000000000000
> R10: 00007faa2aefd700 R11: 0000000000000246 R12: 00007faa2afc20ac
> R13: 00007fff7e6632bf R14: 00007faa2aefd400 R15: 0000000000022000
>  </TASK>
> Modules linked in:
> ---[ end trace 24ec93ff95e4ac3d ]---
> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline]
> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744
> Code: e8 d3 16 d1 ff 48 c7 c6 c0 00 b6 89 48 89 ef e8 94 4e 04 00 0f 0b e8 bd 16 d1 ff 48 c7 c6 60 01 b6 89 48 89 ef e8 7e 4e 04 00 <0f> 0b e8 a7 16 d1 ff 48 c7 c6 a0 01 b6 89 4c 89 f7 e8 68 4e 04 00
> RSP: 0018:ffffc90002b6f7b8 EFLAGS: 00010293
> RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> RDX: ffff888019619d00 RSI: ffffffff81a68c12 RDI: 0000000000000003
> RBP: ffffea0001bdc2c0 R08: 0000000000000029 R09: 00000000ffffffff
> R10: ffffffff8903e29f R11: 00000000ffffffff R12: 00000000ffffffff
> R13: 00000000ffffea00 R14: ffffc90002b6fb30 R15: ffffea0001bd8001
> FS:  00007faa2aefd700(0000) GS:ffff8880b9d00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00007fff7e663318 CR3: 0000000018c6e000 CR4: 00000000003506e0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> 
> The reproducer was trying to reading /proc/$PID/smaps when calling
> MADV_FREE at the mean time.  MADV_FREE may split THPs if it is called
> for partial THP.  It may trigger the below race:
> 
>          CPU A                         CPU B
>          -----                         -----
> smaps walk:                      MADV_FREE:
> page_mapcount()
>   PageCompound()
>                                  split_huge_page()
>   page = compound_head(page)
>   PageDoubleMap(page)
> 
> When calling PageDoubleMap() this page is not a tail page of THP anymore
> so the BUG is triggered.
> 
> This could be fixed by elevated refcount of the page before calling
> mapcount, but it prevents from counting migration entries, and it seems
> overkilling because the race just could happen when PMD is split so all
> PTE entries of tail pages are actually migration entries, and
> smaps_account() does treat migration entries as mapcount == 1 as Kirill
> pointed out.
> 
> Add a new parameter for smaps_account() to tell this entry is migration
> entry then skip calling page_mapcount().  Don't skip getting mapcount for
> device private entries since they do track references with mapcount.
> 
> Pagemap also has the similar issue although it was not reported.  Fixed
> it as well.
> 
> Fixes: e9b61f19858a ("thp: reintroduce split_huge_page()")
> Reported-by: syzbot+1f52b3a18d5633fa7f82@syzkaller.appspotmail.com
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Cc: Jann Horn <jannh@google.com>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Yang Shi <shy828301@gmail.com>
> ---
> v4: * s/Treated/Treat per David
>     * Collected acked-by tag from David
> v3: * Fixed the fix tag, the one used by v2 was not accurate
>     * Added comment about the risk calling page_mapcount() per David
>     * Fix pagemap
> v2: * Added proper fix tag per Jann Horn
>     * Rebased to the latest linus's tree
> 
>  fs/proc/task_mmu.c | 38 ++++++++++++++++++++++++++++++--------
>  1 file changed, 30 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index 18f8c3acbb85..bc2f46033231 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -440,7 +440,8 @@ static void smaps_page_accumulate(struct mem_size_stats *mss,
>  }
>  
>  static void smaps_account(struct mem_size_stats *mss, struct page *page,
> -		bool compound, bool young, bool dirty, bool locked)
> +		bool compound, bool young, bool dirty, bool locked,
> +		bool migration)
>  {
>  	int i, nr = compound ? compound_nr(page) : 1;
>  	unsigned long size = nr * PAGE_SIZE;
> @@ -467,8 +468,15 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
>  	 * page_count(page) == 1 guarantees the page is mapped exactly once.
>  	 * If any subpage of the compound page mapped with PTE it would elevate
>  	 * page_count().
> +	 *
> +	 * The page_mapcount() is called to get a snapshot of the mapcount.
> +	 * Without holding the page lock this snapshot can be slightly wrong as
> +	 * we cannot always read the mapcount atomically.  It is not safe to
> +	 * call page_mapcount() even with PTL held if the page is not mapped,
> +	 * especially for migration entries.  Treat regular migration entries
> +	 * as mapcount == 1.
>  	 */
> -	if (page_count(page) == 1) {
> +	if ((page_count(page) == 1) || migration) {

Since this is now apparently a CVE-2023-1582 for whatever RHeasons...

wonder if the patch actually works as intended when
(page_count() || migration) is in this particular order and not the other one?

>  		smaps_page_accumulate(mss, page, size, size << PSS_SHIFT, dirty,
>  			locked, true);
>  		return;
> @@ -517,6 +525,7 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  	struct vm_area_struct *vma = walk->vma;
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>  	struct page *page = NULL;
> +	bool migration = false;
>  
>  	if (pte_present(*pte)) {
>  		page = vm_normal_page(vma, addr, *pte);
> @@ -536,8 +545,11 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  			} else {
>  				mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
>  			}
> -		} else if (is_pfn_swap_entry(swpent))
> +		} else if (is_pfn_swap_entry(swpent)) {
> +			if (is_migration_entry(swpent))
> +				migration = true;
>  			page = pfn_swap_entry_to_page(swpent);
> +		}
>  	} else {
>  		smaps_pte_hole_lookup(addr, walk);
>  		return;
> @@ -546,7 +558,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
>  	if (!page)
>  		return;
>  
> -	smaps_account(mss, page, false, pte_young(*pte), pte_dirty(*pte), locked);
> +	smaps_account(mss, page, false, pte_young(*pte), pte_dirty(*pte),
> +		      locked, migration);
>  }
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
> @@ -557,6 +570,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	struct vm_area_struct *vma = walk->vma;
>  	bool locked = !!(vma->vm_flags & VM_LOCKED);
>  	struct page *page = NULL;
> +	bool migration = false;
>  
>  	if (pmd_present(*pmd)) {
>  		/* FOLL_DUMP will return -EFAULT on huge zero page */
> @@ -564,8 +578,10 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  	} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
>  		swp_entry_t entry = pmd_to_swp_entry(*pmd);
>  
> -		if (is_migration_entry(entry))
> +		if (is_migration_entry(entry)) {
> +			migration = true;
>  			page = pfn_swap_entry_to_page(entry);
> +		}
>  	}
>  	if (IS_ERR_OR_NULL(page))
>  		return;
> @@ -577,7 +593,9 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
>  		/* pass */;
>  	else
>  		mss->file_thp += HPAGE_PMD_SIZE;
> -	smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd), locked);
> +
> +	smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd),
> +		      locked, migration);
>  }
>  #else
>  static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
> @@ -1378,6 +1396,7 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>  {
>  	u64 frame = 0, flags = 0;
>  	struct page *page = NULL;
> +	bool migration = false;
>  
>  	if (pte_present(pte)) {
>  		if (pm->show_pfn)
> @@ -1399,13 +1418,14 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm,
>  			frame = swp_type(entry) |
>  				(swp_offset(entry) << MAX_SWAPFILES_SHIFT);
>  		flags |= PM_SWAP;
> +		migration = is_migration_entry(entry);
>  		if (is_pfn_swap_entry(entry))
>  			page = pfn_swap_entry_to_page(entry);
>  	}
>  
>  	if (page && !PageAnon(page))
>  		flags |= PM_FILE;
> -	if (page && page_mapcount(page) == 1)
> +	if (page && !migration && page_mapcount(page) == 1)
>  		flags |= PM_MMAP_EXCLUSIVE;
>  	if (vma->vm_flags & VM_SOFTDIRTY)
>  		flags |= PM_SOFT_DIRTY;
> @@ -1421,6 +1441,7 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  	spinlock_t *ptl;
>  	pte_t *pte, *orig_pte;
>  	int err = 0;
> +	bool migration = false;
>  
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  	ptl = pmd_trans_huge_lock(pmdp, vma);
> @@ -1461,11 +1482,12 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  			if (pmd_swp_uffd_wp(pmd))
>  				flags |= PM_UFFD_WP;
>  			VM_BUG_ON(!is_pmd_migration_entry(pmd));
> +			migration = is_migration_entry(entry);
>  			page = pfn_swap_entry_to_page(entry);
>  		}
>  #endif
>  
> -		if (page && page_mapcount(page) == 1)
> +		if (page && !migration && page_mapcount(page) == 1)
>  			flags |= PM_MMAP_EXCLUSIVE;
>  
>  		for (; addr != end; addr += PAGE_SIZE) {