[PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
VMA. This is unreliable as ->mmap may not set ->vm_ops.

False-positive vma_is_anonymous() may lead to crashes:

	next ffff8801ce5e7040 prev ffff8801d20eca50 mm ffff88019c1e13c0
	prot 27 anon_vma ffff88019680cdd8 vm_ops 0000000000000000
	pgoff 0 file ffff8801b2ec2d00 private_data 0000000000000000
	flags: 0xff(read|write|exec|shared|mayread|maywrite|mayexec|mayshare)
	------------[ cut here ]------------
	kernel BUG at mm/memory.c:1422!
	invalid opcode: 0000 [#1] SMP KASAN
	CPU: 0 PID: 18486 Comm: syz-executor3 Not tainted 4.18.0-rc3+ #136
	Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google
	01/01/2011
	RIP: 0010:zap_pmd_range mm/memory.c:1421 [inline]
	RIP: 0010:zap_pud_range mm/memory.c:1466 [inline]
	RIP: 0010:zap_p4d_range mm/memory.c:1487 [inline]
	RIP: 0010:unmap_page_range+0x1c18/0x2220 mm/memory.c:1508
	Code: ff 31 ff 4c 89 e6 42 c6 04 33 f8 e8 92 dd d0 ff 4d 85 e4 0f 85 4a eb ff
	ff e8 54 dc d0 ff 48 8b bd 10 fc ff ff e8 82 95 fe ff <0f> 0b e8 41 dc d0 ff
	0f 0b 4c 89 ad 18 fc ff ff c7 85 7c fb ff ff
	RSP: 0018:ffff8801b0587330 EFLAGS: 00010286
	RAX: 000000000000013c RBX: 1ffff100360b0e9c RCX: ffffc90002620000
	RDX: 0000000000000000 RSI: ffffffff81631851 RDI: 0000000000000001
	RBP: ffff8801b05877c8 R08: ffff880199d40300 R09: ffffed003b5c4fc0
	R10: ffffed003b5c4fc0 R11: ffff8801dae27e07 R12: 0000000000000000
	R13: ffff88019c1e13c0 R14: dffffc0000000000 R15: 0000000020e01000
	FS:  00007fca32251700(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
	CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
	CR2: 00007f04c540d000 CR3: 00000001ac1f0000 CR4: 00000000001426f0
	DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
	DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
	Call Trace:
	 unmap_single_vma+0x1a0/0x310 mm/memory.c:1553
	 zap_page_range_single+0x3cc/0x580 mm/memory.c:1644
	 unmap_mapping_range_vma mm/memory.c:2792 [inline]
	 unmap_mapping_range_tree mm/memory.c:2813 [inline]
	 unmap_mapping_pages+0x3a7/0x5b0 mm/memory.c:2845
	 unmap_mapping_range+0x48/0x60 mm/memory.c:2880
	 truncate_pagecache+0x54/0x90 mm/truncate.c:800
	 truncate_setsize+0x70/0xb0 mm/truncate.c:826
	 simple_setattr+0xe9/0x110 fs/libfs.c:409
	 notify_change+0xf13/0x10f0 fs/attr.c:335
	 do_truncate+0x1ac/0x2b0 fs/open.c:63
	 do_sys_ftruncate+0x492/0x560 fs/open.c:205
	 __do_sys_ftruncate fs/open.c:215 [inline]
	 __se_sys_ftruncate fs/open.c:213 [inline]
	 __x64_sys_ftruncate+0x59/0x80 fs/open.c:213
	 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
	 entry_SYSCALL_64_after_hwframe+0x49/0xbe

Reproducer:

	#include <stdio.h>
	#include <stddef.h>
	#include <stdint.h>
	#include <stdlib.h>
	#include <string.h>
	#include <sys/types.h>
	#include <sys/stat.h>
	#include <sys/ioctl.h>
	#include <sys/mman.h>
	#include <unistd.h>
	#include <fcntl.h>

	#define KCOV_INIT_TRACE			_IOR('c', 1, unsigned long)
	#define KCOV_ENABLE			_IO('c', 100)
	#define KCOV_DISABLE			_IO('c', 101)
	#define COVER_SIZE			(1024<<10)

	#define KCOV_TRACE_PC  0
	#define KCOV_TRACE_CMP 1

	int main(int argc, char **argv)
	{
		int fd;
		unsigned long *cover;

		system("mount -t debugfs none /sys/kernel/debug");
		fd = open("/sys/kernel/debug/kcov", O_RDWR);
		ioctl(fd, KCOV_INIT_TRACE, COVER_SIZE);
		cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long),
				PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
		munmap(cover, COVER_SIZE * sizeof(unsigned long));
		cover = mmap(NULL, COVER_SIZE * sizeof(unsigned long),
				PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
		memset(cover, 0, COVER_SIZE * sizeof(unsigned long));
		ftruncate(fd, 3UL << 20);
		return 0;
	}

This can be fixed by assigning anonymous VMAs own vm_ops and not relying
on it being NULL.

If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reported-by: syzbot+3f84280d52be9b7083cc@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
---
 drivers/char/mem.c   |  1 +
 fs/exec.c            |  1 +
 fs/hugetlbfs/inode.c |  1 +
 include/linux/mm.h   |  5 ++++-
 mm/khugepaged.c      |  4 ++--
 mm/mmap.c            | 13 +++++++++++++
 mm/shmem.c           |  1 +
 7 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index ffeb60d3434c..f0a8b0b1768b 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -708,6 +708,7 @@ static int mmap_zero(struct file *file, struct vm_area_struct *vma)
 #endif
 	if (vma->vm_flags & VM_SHARED)
 		return shmem_zero_setup(vma);
+	vma->vm_ops = &anon_vm_ops;
 	return 0;
 }
 
diff --git a/fs/exec.c b/fs/exec.c
index 2d4e0075bd24..a1a246062561 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -307,6 +307,7 @@ static int __bprm_mm_init(struct linux_binprm *bprm)
 	 * configured yet.
 	 */
 	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
+	vma->vm_ops = &anon_vm_ops;
 	vma->vm_end = STACK_TOP_MAX;
 	vma->vm_start = vma->vm_end - PAGE_SIZE;
 	vma->vm_flags = VM_SOFTDIRTY | VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index d508c7844681..5ff73b1398ad 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -597,6 +597,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
 	memset(&pseudo_vma, 0, sizeof(struct vm_area_struct));
 	pseudo_vma.vm_flags = (VM_HUGETLB | VM_MAYSHARE | VM_SHARED);
 	pseudo_vma.vm_file = file;
+	pseudo_vma.vm_ops = &dummy_vm_ops;
 
 	for (index = start; index < end; index++) {
 		/*
diff --git a/include/linux/mm.h b/include/linux/mm.h
index a0fbb9ffe380..9a35362bbc92 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1536,9 +1536,12 @@ int clear_page_dirty_for_io(struct page *page);
 
 int get_cmdline(struct task_struct *task, char *buffer, int buflen);
 
+extern const struct vm_operations_struct anon_vm_ops;
+extern const struct vm_operations_struct dummy_vm_ops;
+
 static inline bool vma_is_anonymous(struct vm_area_struct *vma)
 {
-	return !vma->vm_ops;
+	return vma->vm_ops == &anon_vm_ops;
 }
 
 #ifdef CONFIG_SHMEM
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index d7b2a4bf8671..5ae34097aed1 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -440,7 +440,7 @@ int khugepaged_enter_vma_merge(struct vm_area_struct *vma,
 		 * page fault if needed.
 		 */
 		return 0;
-	if (vma->vm_ops || (vm_flags & VM_NO_KHUGEPAGED))
+	if (!vma_is_anonymous(vma) || (vm_flags & VM_NO_KHUGEPAGED))
 		/* khugepaged not yet working on file or special mappings */
 		return 0;
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
@@ -831,7 +831,7 @@ static bool hugepage_vma_check(struct vm_area_struct *vma)
 		return IS_ALIGNED((vma->vm_start >> PAGE_SHIFT) - vma->vm_pgoff,
 				HPAGE_PMD_NR);
 	}
-	if (!vma->anon_vma || vma->vm_ops)
+	if (!vma->anon_vma || !vma_is_anonymous(vma))
 		return false;
 	if (is_vma_temporary_stack(vma))
 		return false;
diff --git a/mm/mmap.c b/mm/mmap.c
index d1eb87ef4b1a..0729ed06b01c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -71,6 +71,9 @@ int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
 static bool ignore_rlimit_data;
 core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
 
+const struct vm_operations_struct anon_vm_ops = {};
+const struct vm_operations_struct dummy_vm_ops = {};
+
 static void unmap_region(struct mm_struct *mm,
 		struct vm_area_struct *vma, struct vm_area_struct *prev,
 		unsigned long start, unsigned long end);
@@ -561,6 +564,8 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
 void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
 		struct rb_node **rb_link, struct rb_node *rb_parent)
 {
+	WARN_ONCE(!vma->vm_ops, "missing vma->vm_ops");
+
 	/* Update tracking information for the gap following the new vma. */
 	if (vma->vm_next)
 		vma_gap_update(vma->vm_next);
@@ -1774,12 +1779,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		 */
 		WARN_ON_ONCE(addr != vma->vm_start);
 
+		/* All mappings must have ->vm_ops set */
+		if (!vma->vm_ops)
+			vma->vm_ops = &dummy_vm_ops;
+
 		addr = vma->vm_start;
 		vm_flags = vma->vm_flags;
 	} else if (vm_flags & VM_SHARED) {
 		error = shmem_zero_setup(vma);
 		if (error)
 			goto free_vma;
+	} else {
+		/* vma_is_anonymous() relies on this. */
+		vma->vm_ops = &anon_vm_ops;
 	}
 
 	vma_link(mm, vma, prev, rb_link, rb_parent);
@@ -2999,6 +3011,7 @@ static int do_brk_flags(unsigned long addr, unsigned long request, unsigned long
 
 	INIT_LIST_HEAD(&vma->anon_vma_chain);
 	vma->vm_mm = mm;
+	vma->vm_ops = &anon_vm_ops;
 	vma->vm_start = addr;
 	vma->vm_end = addr + len;
 	vma->vm_pgoff = pgoff;
diff --git a/mm/shmem.c b/mm/shmem.c
index 2cab84403055..db5c319161ca 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1424,6 +1424,7 @@ static void shmem_pseudo_vma_init(struct vm_area_struct *vma,
 	/* Bias interleave by inode number to distribute better across nodes */
 	vma->vm_pgoff = index + info->vfs_inode.i_ino;
 	vma->vm_policy = mpol_shared_policy_lookup(&info->policy, index);
+	vma->vm_ops = &dummy_vm_ops;
 }
 
 static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
-- 
2.18.0

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Andrew Morton]

On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> VMA. This is unreliable as ->mmap may not set ->vm_ops.
> 
> False-positive vma_is_anonymous() may lead to crashes:
> 
> ...
> 
> This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> on it being NULL.
> 
> If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.

Is there a smaller, simpler fix which we can use for backporting
purposes and save the larger rework for development kernels?

>
> ...
>
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -71,6 +71,9 @@ int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
>  static bool ignore_rlimit_data;
>  core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
>  
> +const struct vm_operations_struct anon_vm_ops = {};
> +const struct vm_operations_struct dummy_vm_ops = {};

Some nice comments here would be useful.  Especially for dummy_vm_ops. 
Why does it exist, what is its role, etc.

>  static void unmap_region(struct mm_struct *mm,
>  		struct vm_area_struct *vma, struct vm_area_struct *prev,
>  		unsigned long start, unsigned long end);
> @@ -561,6 +564,8 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
>  void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
>  		struct rb_node **rb_link, struct rb_node *rb_parent)
>  {
> +	WARN_ONCE(!vma->vm_ops, "missing vma->vm_ops");
> +
>  	/* Update tracking information for the gap following the new vma. */
>  	if (vma->vm_next)
>  		vma_gap_update(vma->vm_next);
> @@ -1774,12 +1779,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		 */
>  		WARN_ON_ONCE(addr != vma->vm_start);
>  
> +		/* All mappings must have ->vm_ops set */
> +		if (!vma->vm_ops)
> +			vma->vm_ops = &dummy_vm_ops;

Can this happen?  Can we make it a rule that file_operations.mmap(vma)
must initialize vma->vm_ops?  Should we have a WARN here to detect when
the fs implementation failed to do that?

>  		addr = vma->vm_start;
>  		vm_flags = vma->vm_flags;
>  	} else if (vm_flags & VM_SHARED) {
>  		error = shmem_zero_setup(vma);
>  		if (error)
>  			goto free_vma;
> +	} else {
> +		/* vma_is_anonymous() relies on this. */
> +		vma->vm_ops = &anon_vm_ops;
>  	}
>  
>  	vma_link(mm, vma, prev, rb_link, rb_parent);
> ...
>

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Tue, Jul 10, 2018 at 01:48:58PM -0700, Andrew Morton wrote:
> On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> 
> > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > 
> > False-positive vma_is_anonymous() may lead to crashes:
> > 
> > ...
> > 
> > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > on it being NULL.
> > 
> > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> 
> Is there a smaller, simpler fix which we can use for backporting
> purposes and save the larger rework for development kernels?

I've tried to move dummy_vm_ops stuff into a separate patch, but it didn't
workaround.

In some cases (like in create_huge_pmd()/wp_huge_pmd()) we rely on
vma_is_anonymous() to guarantee that ->vm_ops is non-NULL. But with new
implementation of the helper there's no such guarantee. And I see crash in
create_huge_pmd().

We can add explicit ->vm_ops check in such places. But it's more risky.
I may miss some instances. dummy_vm_ops should be safer here.

I think it's better to backport whole patch.

> 
> >
> > ...
> >
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -71,6 +71,9 @@ int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
> >  static bool ignore_rlimit_data;
> >  core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
> >  
> > +const struct vm_operations_struct anon_vm_ops = {};
> > +const struct vm_operations_struct dummy_vm_ops = {};
> 
> Some nice comments here would be useful.  Especially for dummy_vm_ops. 
> Why does it exist, what is its role, etc.

Fixup is below.

> >  static void unmap_region(struct mm_struct *mm,
> >  		struct vm_area_struct *vma, struct vm_area_struct *prev,
> >  		unsigned long start, unsigned long end);
> > @@ -561,6 +564,8 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
> >  void __vma_link_rb(struct mm_struct *mm, struct vm_area_struct *vma,
> >  		struct rb_node **rb_link, struct rb_node *rb_parent)
> >  {
> > +	WARN_ONCE(!vma->vm_ops, "missing vma->vm_ops");
> > +
> >  	/* Update tracking information for the gap following the new vma. */
> >  	if (vma->vm_next)
> >  		vma_gap_update(vma->vm_next);
> > @@ -1774,12 +1779,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> >  		 */
> >  		WARN_ON_ONCE(addr != vma->vm_start);
> >  
> > +		/* All mappings must have ->vm_ops set */
> > +		if (!vma->vm_ops)
> > +			vma->vm_ops = &dummy_vm_ops;
> 
> Can this happen?  Can we make it a rule that file_operations.mmap(vma)
> must initialize vma->vm_ops?  Should we have a WARN here to detect when
> the fs implementation failed to do that?

Yes, it can happen. KCOV doesn't set it now. And I'm pretty sure some
drivers do not set it too.

We can add warning here. But I'm not sure what value it would have.
It's perfectly fine to have no need in any of vm operations. Silently set
it to dummy_vm_ops should be good enough here.

> >  		addr = vma->vm_start;
> >  		vm_flags = vma->vm_flags;
> >  	} else if (vm_flags & VM_SHARED) {
> >  		error = shmem_zero_setup(vma);
> >  		if (error)
> >  			goto free_vma;
> > +	} else {
> > +		/* vma_is_anonymous() relies on this. */
> +		vma->vm_ops = &anon_vm_ops;
> >  	}
> >  
> >  	vma_link(mm, vma, prev, rb_link, rb_parent);
> > ...
> >
> 

diff --git a/mm/mmap.c b/mm/mmap.c
index 0729ed06b01c..6f59ade58fa7 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -71,7 +71,16 @@ int mmap_rnd_compat_bits __read_mostly = CONFIG_ARCH_MMAP_RND_COMPAT_BITS;
 static bool ignore_rlimit_data;
 core_param(ignore_rlimit_data, ignore_rlimit_data, bool, 0644);
 
+/*
+ * All anonymous VMAs have ->vm_ops set to anon_vm_ops.
+ * vma_is_anonymous() reiles on anon_vm_ops to detect anonymous VMA.
+ */
 const struct vm_operations_struct anon_vm_ops = {};
+
+/*
+ * All VMAs have to have ->vm_ops set. dummy_vm_ops can be used if the VMA
+ * doesn't need to handle any of the operations.
+ */
 const struct vm_operations_struct dummy_vm_ops = {};
 
 static void unmap_region(struct mm_struct *mm,
-- 
 Kirill A. Shutemov

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Michal Hocko]

On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> 
> > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > 
> > False-positive vma_is_anonymous() may lead to crashes:
> > 
> > ...
> > 
> > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > on it being NULL.
> > 
> > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> 
> Is there a smaller, simpler fix which we can use for backporting
> purposes and save the larger rework for development kernels?

Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
for all users who do not initialize it in their mmap callbacks?
Basically have a sanity check&fixup in call_mmap?
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > 
> > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > 
> > > False-positive vma_is_anonymous() may lead to crashes:
> > > 
> > > ...
> > > 
> > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > on it being NULL.
> > > 
> > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > 
> > Is there a smaller, simpler fix which we can use for backporting
> > purposes and save the larger rework for development kernels?
> 
> Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> for all users who do not initialize it in their mmap callbacks?
> Basically have a sanity check&fixup in call_mmap?

As I said, there's a corner case of MAP_PRIVATE of /dev/zero. It has to
produce anonymous VMA, but in map_region() we cannot distinguish it from
broken ->mmap handler.

See my attempt

	6dc296e7df4c ("mm: make sure all file VMAs have ->vm_ops set")

and it's revert

	 28c553d0aa0a ("revert "mm: make sure all file VMAs have ->vm_ops set"")

-- 
 Kirill A. Shutemov

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Michal Hocko]

On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > 
> > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > 
> > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > 
> > > > ...
> > > > 
> > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > on it being NULL.
> > > > 
> > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > 
> > > Is there a smaller, simpler fix which we can use for backporting
> > > purposes and save the larger rework for development kernels?
> > 
> > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > for all users who do not initialize it in their mmap callbacks?
> > Basically have a sanity check&fixup in call_mmap?
> 
> As I said, there's a corner case of MAP_PRIVATE of /dev/zero.

This is really creative. I really didn't think about that. I am
wondering whether this really has to be handled as a private anonymous
mapping implicitly. Why does vma_is_anonymous has to succeed for these
mappings? Why cannot we simply handle it as any other file backed
PRIVATE mapping?

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > 
> > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > 
> > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > 
> > > > > ...
> > > > > 
> > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > on it being NULL.
> > > > > 
> > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > 
> > > > Is there a smaller, simpler fix which we can use for backporting
> > > > purposes and save the larger rework for development kernels?
> > > 
> > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > for all users who do not initialize it in their mmap callbacks?
> > > Basically have a sanity check&fixup in call_mmap?
> > 
> > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> 
> This is really creative. I really didn't think about that. I am
> wondering whether this really has to be handled as a private anonymous
> mapping implicitly. Why does vma_is_anonymous has to succeed for these
> mappings? Why cannot we simply handle it as any other file backed
> PRIVATE mapping?

Because it's established way to create anonymous mappings in Linux.
And we cannot break the semantics.

-- 
 Kirill A. Shutemov

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Michal Hocko]

On Mon 16-07-18 17:47:39, Kirill A. Shutemov wrote:
> On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> > On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > > 
> > > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > > 
> > > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > > 
> > > > > > ...
> > > > > > 
> > > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > > on it being NULL.
> > > > > > 
> > > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > > 
> > > > > Is there a smaller, simpler fix which we can use for backporting
> > > > > purposes and save the larger rework for development kernels?
> > > > 
> > > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > > for all users who do not initialize it in their mmap callbacks?
> > > > Basically have a sanity check&fixup in call_mmap?
> > > 
> > > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> > 
> > This is really creative. I really didn't think about that. I am
> > wondering whether this really has to be handled as a private anonymous
> > mapping implicitly. Why does vma_is_anonymous has to succeed for these
> > mappings? Why cannot we simply handle it as any other file backed
> > PRIVATE mapping?
> 
> Because it's established way to create anonymous mappings in Linux.
> And we cannot break the semantics.

How exactly would semantic break? You would still get zero pages on read
faults and anonymous pages on CoW. So basically the same thing as for
any other file backed MAP_PRIVATE mapping.
-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Mon, Jul 16, 2018 at 07:40:42PM +0200, Michal Hocko wrote:
> On Mon 16-07-18 17:47:39, Kirill A. Shutemov wrote:
> > On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> > > On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > > > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > > > 
> > > > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > > > 
> > > > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > > > 
> > > > > > > ...
> > > > > > > 
> > > > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > > > on it being NULL.
> > > > > > > 
> > > > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > > > 
> > > > > > Is there a smaller, simpler fix which we can use for backporting
> > > > > > purposes and save the larger rework for development kernels?
> > > > > 
> > > > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > > > for all users who do not initialize it in their mmap callbacks?
> > > > > Basically have a sanity check&fixup in call_mmap?
> > > > 
> > > > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> > > 
> > > This is really creative. I really didn't think about that. I am
> > > wondering whether this really has to be handled as a private anonymous
> > > mapping implicitly. Why does vma_is_anonymous has to succeed for these
> > > mappings? Why cannot we simply handle it as any other file backed
> > > PRIVATE mapping?
> > 
> > Because it's established way to create anonymous mappings in Linux.
> > And we cannot break the semantics.
> 
> How exactly would semantic break? You would still get zero pages on read
> faults and anonymous pages on CoW. So basically the same thing as for
> any other file backed MAP_PRIVATE mapping.

You are wrong about zero page. And you won't get THP. And I'm sure there's
more differences. Just grep for vma_is_anonymous().

-- 
 Kirill A. Shutemov

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Michal Hocko]

On Mon 16-07-18 23:38:46, Kirill A. Shutemov wrote:
> On Mon, Jul 16, 2018 at 07:40:42PM +0200, Michal Hocko wrote:
> > On Mon 16-07-18 17:47:39, Kirill A. Shutemov wrote:
> > > On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> > > > On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > > > > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > > > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > > > > 
> > > > > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > > > > 
> > > > > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > > > > 
> > > > > > > > ...
> > > > > > > > 
> > > > > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > > > > on it being NULL.
> > > > > > > > 
> > > > > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > > > > 
> > > > > > > Is there a smaller, simpler fix which we can use for backporting
> > > > > > > purposes and save the larger rework for development kernels?
> > > > > > 
> > > > > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > > > > for all users who do not initialize it in their mmap callbacks?
> > > > > > Basically have a sanity check&fixup in call_mmap?
> > > > > 
> > > > > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> > > > 
> > > > This is really creative. I really didn't think about that. I am
> > > > wondering whether this really has to be handled as a private anonymous
> > > > mapping implicitly. Why does vma_is_anonymous has to succeed for these
> > > > mappings? Why cannot we simply handle it as any other file backed
> > > > PRIVATE mapping?
> > > 
> > > Because it's established way to create anonymous mappings in Linux.
> > > And we cannot break the semantics.
> > 
> > How exactly would semantic break? You would still get zero pages on read
> > faults and anonymous pages on CoW. So basically the same thing as for
> > any other file backed MAP_PRIVATE mapping.
> 
> You are wrong about zero page.

Well, if we redirect ->fault to do_anonymous_page and

> And you won't get THP.

huge_fault to do_huge_pmd_anonymous_page then we should emulate the
standard anonymous mapping.

> And I'm sure there's more differences. Just grep for
> vma_is_anonymous().

I am sorry to push on this but if we have one odd case I would rather
handle it and have a simple _rule_ that every mmap provide _has_ to
provide vm_ops and have a trivial fix up at a single place rather than
patch a subtle placeholders you were proposing.

I will not insist of course but this looks less fragile to me.

-- 
Michal Hocko
SUSE Labs

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Kirill A. Shutemov]

On Tue, Jul 17, 2018 at 09:00:53AM +0000, Michal Hocko wrote:
> On Mon 16-07-18 23:38:46, Kirill A. Shutemov wrote:
> > On Mon, Jul 16, 2018 at 07:40:42PM +0200, Michal Hocko wrote:
> > > On Mon 16-07-18 17:47:39, Kirill A. Shutemov wrote:
> > > > On Mon, Jul 16, 2018 at 04:22:45PM +0200, Michal Hocko wrote:
> > > > > On Mon 16-07-18 17:04:41, Kirill A. Shutemov wrote:
> > > > > > On Mon, Jul 16, 2018 at 01:30:28PM +0000, Michal Hocko wrote:
> > > > > > > On Tue 10-07-18 13:48:58, Andrew Morton wrote:
> > > > > > > > On Tue, 10 Jul 2018 16:48:20 +0300 "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:
> > > > > > > > 
> > > > > > > > > vma_is_anonymous() relies on ->vm_ops being NULL to detect anonymous
> > > > > > > > > VMA. This is unreliable as ->mmap may not set ->vm_ops.
> > > > > > > > > 
> > > > > > > > > False-positive vma_is_anonymous() may lead to crashes:
> > > > > > > > > 
> > > > > > > > > ...
> > > > > > > > > 
> > > > > > > > > This can be fixed by assigning anonymous VMAs own vm_ops and not relying
> > > > > > > > > on it being NULL.
> > > > > > > > > 
> > > > > > > > > If ->mmap() failed to set ->vm_ops, mmap_region() will set it to
> > > > > > > > > dummy_vm_ops. This way we will have non-NULL ->vm_ops for all VMAs.
> > > > > > > > 
> > > > > > > > Is there a smaller, simpler fix which we can use for backporting
> > > > > > > > purposes and save the larger rework for development kernels?
> > > > > > > 
> > > > > > > Why cannot we simply keep anon vma with null vm_ops and set dummy_vm_ops
> > > > > > > for all users who do not initialize it in their mmap callbacks?
> > > > > > > Basically have a sanity check&fixup in call_mmap?
> > > > > > 
> > > > > > As I said, there's a corner case of MAP_PRIVATE of /dev/zero.
> > > > > 
> > > > > This is really creative. I really didn't think about that. I am
> > > > > wondering whether this really has to be handled as a private anonymous
> > > > > mapping implicitly. Why does vma_is_anonymous has to succeed for these
> > > > > mappings? Why cannot we simply handle it as any other file backed
> > > > > PRIVATE mapping?
> > > > 
> > > > Because it's established way to create anonymous mappings in Linux.
> > > > And we cannot break the semantics.
> > > 
> > > How exactly would semantic break? You would still get zero pages on read
> > > faults and anonymous pages on CoW. So basically the same thing as for
> > > any other file backed MAP_PRIVATE mapping.
> > 
> > You are wrong about zero page.
> 
> Well, if we redirect ->fault to do_anonymous_page and

Yeah. And it will make write fault to allocate *two* pages. One in
do_anonymous_page() and one in do_cow_fault(). Just no.

We have a reason why anon VMAs handled separately. It's possible to unify
them, but it requires substantial ground work.

> > And you won't get THP.
> 
> huge_fault to do_huge_pmd_anonymous_page then we should emulate the
> standard anonymous mapping.
> 
> > And I'm sure there's more differences. Just grep for
> > vma_is_anonymous().
> 
> I am sorry to push on this but if we have one odd case I would rather
> handle it and have a simple _rule_ that every mmap provide _has_ to
> provide vm_ops and have a trivial fix up at a single place rather than
> patch a subtle placeholders you were proposing.
> 
> I will not insist of course but this looks less fragile to me.

You propose quite a big redesign on how we handle anonymous VMAs.
Feel free to propose the patch(set). But I don't think it would fly for
stable@.

-- 
 Kirill A. Shutemov

Re: [PATCH 1/2] mm: Fix vma_is_anonymous() false-positives

[Michal Hocko]

On Tue 17-07-18 12:30:30, Kirill A. Shutemov wrote:
[...]
> You propose quite a big redesign on how we handle anonymous VMAs.
> Feel free to propose the patch(set). But I don't think it would fly for
> stable@.

OK, fair enough. I thought this would be much easier in the end but I
admit I haven't tried that so I might have underestimated the whole
thing.
-- 
Michal Hocko
SUSE Labs