[PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Konstantin Khlebnikov]

"no other files mapped" requirement from my previous patch
(c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
is too paranoid, it forbids operation even if there mapped one shared-anon vma.

Let's check that current mm->exe_file already unmapped, in this case exe_file
symlink already outdated and its changing is reasonable.

Plus, this patch fixes exit code in case operation success.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Pavel Emelyanov <xemul@parallels.com>
---
 kernel/sys.c |   16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index f0ec44d..eb4c87a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1788,7 +1788,6 @@ SYSCALL_DEFINE1(umask, int, mask)
 #ifdef CONFIG_CHECKPOINT_RESTORE
 static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 {
-	struct vm_area_struct *vma;
 	struct file *exe_file;
 	struct dentry *dentry;
 	int err;
@@ -1816,13 +1815,17 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	down_write(&mm->mmap_sem);
 
 	/*
-	 * Forbid mm->exe_file change if there are mapped other files.
+	 * Forbid mm->exe_file change if old file still mapped.
 	 */
 	err = -EBUSY;
-	for (vma = mm->mmap; vma; vma = vma->vm_next) {
-		if (vma->vm_file && !path_equal(&vma->vm_file->f_path,
-						&exe_file->f_path))
-			goto exit_unlock;
+	if (mm->exe_file) {
+		struct vm_area_struct *vma;
+
+		for (vma = mm->mmap; vma; vma = vma->vm_next)
+			if (vma->vm_file &&
+			    path_equal(&vma->vm_file->f_path,
+				       &mm->exe_file->f_path))
+				goto exit_unlock;
 	}
 
 	/*
@@ -1835,6 +1838,7 @@ static int prctl_set_mm_exe_file(struct mm_struct *mm, unsigned int fd)
 	if (test_and_set_bit(MMF_EXE_FILE_CHANGED, &mm->flags))
 		goto exit_unlock;
 
+	err = 0;
 	set_mm_exe_file(mm, exe_file);
 exit_unlock:
 	up_write(&mm->mmap_sem);

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 12:51:04PM +0400, Konstantin Khlebnikov wrote:
> "no other files mapped" requirement from my previous patch
> (c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
> is too paranoid, it forbids operation even if there mapped one shared-anon vma.
> 
> Let's check that current mm->exe_file already unmapped, in this case exe_file
> symlink already outdated and its changing is reasonable.
> 
> Plus, this patch fixes exit code in case operation success.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>

Thanks, Konstantin, letme test it out...

	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 12:51:04PM +0400, Konstantin Khlebnikov wrote:
> "no other files mapped" requirement from my previous patch
> (c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
> is too paranoid, it forbids operation even if there mapped one shared-anon vma.
> 
> Let's check that current mm->exe_file already unmapped, in this case exe_file
> symlink already outdated and its changing is reasonable.
> 
> Plus, this patch fixes exit code in case operation success.
> 
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Matt Helsley <matthltc@us.ibm.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Pavel Emelyanov <xemul@parallels.com>
> ---

Ack! Thanks again, Konstantin!

	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 01:06:46PM +0400, Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 12:51:04PM +0400, Konstantin Khlebnikov wrote:
> > "no other files mapped" requirement from my previous patch
> > (c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
> > is too paranoid, it forbids operation even if there mapped one shared-anon vma.
> > 
> > Let's check that current mm->exe_file already unmapped, in this case exe_file
> > symlink already outdated and its changing is reasonable.
> > 
> > Plus, this patch fixes exit code in case operation success.
> > 
> > Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> > Reported-by: Cyrill Gorcunov <gorcunov@openvz.org>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: Matt Helsley <matthltc@us.ibm.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Pavel Emelyanov <xemul@parallels.com>
> > ---
> 
> Ack! Thanks again, Konstantin!

Side note: there is a little nit with this patch actually,
because while when we do c/r we do "right things" and unmap
all vm-executable mappings before we set up new exe_file. But
we can't guarantee that some brave soul would not setup
new exe-file just for it's own, then what we migh have

 - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be > 1
 - application calls for prctl_set_mm_exe_file
 - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
 - finally application might call for removed_exe_file_vma

void removed_exe_file_vma(struct mm_struct *mm)
{
	mm->num_exe_file_vmas--;
	if ((mm->num_exe_file_vmas == 0) && mm->exe_file) {
		fput(mm->exe_file);
		mm->exe_file = NULL;
	}

}

and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
thus we get inconsistency in counter.

	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Konstantin Khlebnikov]

Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 01:06:46PM +0400, Cyrill Gorcunov wrote:
>> On Sat, Jun 16, 2012 at 12:51:04PM +0400, Konstantin Khlebnikov wrote:
>>> "no other files mapped" requirement from my previous patch
>>> (c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal)
>>> is too paranoid, it forbids operation even if there mapped one shared-anon vma.
>>>
>>> Let's check that current mm->exe_file already unmapped, in this case exe_file
>>> symlink already outdated and its changing is reasonable.
>>>
>>> Plus, this patch fixes exit code in case operation success.
>>>
>>> Signed-off-by: Konstantin Khlebnikov<khlebnikov@openvz.org>
>>> Reported-by: Cyrill Gorcunov<gorcunov@openvz.org>
>>> Cc: Oleg Nesterov<oleg@redhat.com>
>>> Cc: Matt Helsley<matthltc@us.ibm.com>
>>> Cc: Kees Cook<keescook@chromium.org>
>>> Cc: KOSAKI Motohiro<kosaki.motohiro@jp.fujitsu.com>
>>> Cc: Tejun Heo<tj@kernel.org>
>>> Cc: Pavel Emelyanov<xemul@parallels.com>
>>> ---
>>
>> Ack! Thanks again, Konstantin!
>
> Side note: there is a little nit with this patch actually,
> because while when we do c/r we do "right things" and unmap
> all vm-executable mappings before we set up new exe_file. But
> we can't guarantee that some brave soul would not setup
> new exe-file just for it's own, then what we migh have
>
>   - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be>  1
>   - application calls for prctl_set_mm_exe_file
>   - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
>   - finally application might call for removed_exe_file_vma
>
> void removed_exe_file_vma(struct mm_struct *mm)
> {
> 	mm->num_exe_file_vmas--;
> 	if ((mm->num_exe_file_vmas == 0)&&  mm->exe_file) {
> 		fput(mm->exe_file);
> 		mm->exe_file = NULL;
> 	}
>
> }
>
> and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
> thus we get inconsistency in counter.

No, removed_exe_file_vma() is called only for vma with VM_EXECUTABLE flag,
there no way to get such vma other than sys_execve().
And this brave soul cannot call prctl_set_mm_exe_file() successfully,
just because for vma with VM_EXECUTABLE flag vma->vm_file == mm->exe_file.

Anyway, I plan to get rid of mm->num_exe_file_vmas and VM_EXECUTABLE.

>
> 	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 01:42:23PM +0400, Konstantin Khlebnikov wrote:
> >Side note: there is a little nit with this patch actually,
> >because while when we do c/r we do "right things" and unmap
> >all vm-executable mappings before we set up new exe_file. But
> >we can't guarantee that some brave soul would not setup
> >new exe-file just for it's own, then what we migh have
> >
> >  - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be>  1
> >  - application calls for prctl_set_mm_exe_file
> >  - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
> >  - finally application might call for removed_exe_file_vma
> >
> >void removed_exe_file_vma(struct mm_struct *mm)
> >{
> >	mm->num_exe_file_vmas--;
> >	if ((mm->num_exe_file_vmas == 0)&&  mm->exe_file) {
> >		fput(mm->exe_file);
> >		mm->exe_file = NULL;
> >	}
> >
> >}
> >
> >and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
> >thus we get inconsistency in counter.
> 
> No, removed_exe_file_vma() is called only for vma with VM_EXECUTABLE flag,
> there no way to get such vma other than sys_execve().
> And this brave soul cannot call prctl_set_mm_exe_file() successfully,
> just because for vma with VM_EXECUTABLE flag vma->vm_file == mm->exe_file.
> 
> Anyway, I plan to get rid of mm->num_exe_file_vmas and VM_EXECUTABLE.

Yeah, you've changed !path_equal to path_equal. And yes, getting rid of
num_exe_file_vmas is good idea. Btw, Konstantin, why do we need to
call for path_equal? Maybe we can simply test for mm->exe_file == NULL,
and refuse to change anything if it's not nil value? This will simplify
the code.

	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Konstantin Khlebnikov]

Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 01:42:23PM +0400, Konstantin Khlebnikov wrote:
>>> Side note: there is a little nit with this patch actually,
>>> because while when we do c/r we do "right things" and unmap
>>> all vm-executable mappings before we set up new exe_file. But
>>> we can't guarantee that some brave soul would not setup
>>> new exe-file just for it's own, then what we migh have
>>>
>>>   - mm::exe_file set up and points to some file, moreover num_exe_file_vmas might be>   1
>>>   - application calls for prctl_set_mm_exe_file
>>>   - set_mm_exe_file(mm, exe_file) called, and it drops num_exe_file_vmas to 0
>>>   - finally application might call for removed_exe_file_vma
>>>
>>> void removed_exe_file_vma(struct mm_struct *mm)
>>> {
>>> 	mm->num_exe_file_vmas--;
>>> 	if ((mm->num_exe_file_vmas == 0)&&   mm->exe_file) {
>>> 		fput(mm->exe_file);
>>> 		mm->exe_file = NULL;
>>> 	}
>>>
>>> }
>>>
>>> and it does _not_ test for num_exe_file_vmas being 0 before doing decrement,
>>> thus we get inconsistency in counter.
>>
>> No, removed_exe_file_vma() is called only for vma with VM_EXECUTABLE flag,
>> there no way to get such vma other than sys_execve().
>> And this brave soul cannot call prctl_set_mm_exe_file() successfully,
>> just because for vma with VM_EXECUTABLE flag vma->vm_file == mm->exe_file.
>>
>> Anyway, I plan to get rid of mm->num_exe_file_vmas and VM_EXECUTABLE.
>
> Yeah, you've changed !path_equal to path_equal. And yes, getting rid of
> num_exe_file_vmas is good idea. Btw, Konstantin, why do we need to
> call for path_equal? Maybe we can simply test for mm->exe_file == NULL,
> and refuse to change anything if it's not nil value? This will simplify
> the code.

After removing VM_EXECUTABLE and mm->num_exe_file_vmas mm->exe_file
will never becomes NULL automatically. Patch for this not commited yet,
but I hope it will be in 3.6.

>
> 	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 07:38:29PM +0400, Konstantin Khlebnikov wrote:
> >Yeah, you've changed !path_equal to path_equal. And yes, getting rid of
> >num_exe_file_vmas is good idea. Btw, Konstantin, why do we need to
> >call for path_equal? Maybe we can simply test for mm->exe_file == NULL,
> >and refuse to change anything if it's not nil value? This will simplify
> >the code.
> 
> After removing VM_EXECUTABLE and mm->num_exe_file_vmas mm->exe_file
> will never becomes NULL automatically. Patch for this not commited yet,
> but I hope it will be in 3.6.

OK, lets stick with current patch then.

	Cyrill

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Re: [PATCH 3.5] c/r: prctl: less paranoid prctl_set_mm_exe_file()

[Cyrill Gorcunov]

On Sat, Jun 16, 2012 at 07:44:24PM +0400, Cyrill Gorcunov wrote:
> On Sat, Jun 16, 2012 at 07:38:29PM +0400, Konstantin Khlebnikov wrote:
> > >Yeah, you've changed !path_equal to path_equal. And yes, getting rid of
> > >num_exe_file_vmas is good idea. Btw, Konstantin, why do we need to
> > >call for path_equal? Maybe we can simply test for mm->exe_file == NULL,
> > >and refuse to change anything if it's not nil value? This will simplify
> > >the code.
> > 
> > After removing VM_EXECUTABLE and mm->num_exe_file_vmas mm->exe_file
> > will never becomes NULL automatically. Patch for this not commited yet,
> > but I hope it will be in 3.6.
> 
> OK, lets stick with current patch then.

To clarify

Tested-by: Cyrill Gorcunov <gorcunov@openvz.org>

Andrew, could you please pick up this bugfix. It's critical for us.

P.S. Together with patch https://lkml.org/lkml/2012/6/15/220 it'll be
last changes to prctl in a sake of c/r I think. Would be cool to have
both bugfixes in 3.5.

	Cyrill

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