fs/binfmt_elf_fdpic.c: fix brk area overlap with stack on NOMMU

Commit Message

Rich Felker Aug. 20, 2015, 7:11 p.m. UTC

From: Rich Felker <[e-mail]>

On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
overlap with all but the last PAGE_SIZE bytes of the stack. This leads
to catastrophic memory reuse/corruption if brk is used. Fix by setting
the brk area to zero size to disable its use.

Signed-off-by: Rich Felker <[e-mail]>
---

There is no reason for the kernel to be providing a brk area at all on
NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
NOMMU targets, and musl libc goes out of its way to avoid using brk
that might run into the stack.

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Comments

Greg Ungerer Aug. 26, 2015, 1:26 a.m. UTC | #1

Hi Rich,

On 21/08/15 05:11, Rich Felker wrote:
> From: Rich Felker <[e-mail]>
> 
> On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
> overlap with all but the last PAGE_SIZE bytes of the stack. This leads
> to catastrophic memory reuse/corruption if brk is used. Fix by setting
> the brk area to zero size to disable its use.
> 
> Signed-off-by: Rich Felker <[e-mail]>

It would make sense to run this by David Howells <[e-mail]>,
I think he wrote this code (added to CC list).

I have no problem with it, so:

Acked-by: Greg Ungerer <[e-mail]>

> ---
> 
> There is no reason for the kernel to be providing a brk area at all on
> NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
> NOMMU targets, and musl libc goes out of its way to avoid using brk
> that might run into the stack.

I recall a long time back someone was playing with the idea of setting
the brk to the unused parts of the last data area page. (Somewhat like
this code seems to be trying). That scheme still allocated the full
requested stack size (IIRC) though. And that would have been on bFLT
executables. Anyway, just some historical reference, not really
relevant now.

Regards
Greg



> --- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
> +++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
> @@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
>  		PAGE_ALIGN(current->mm->start_brk);
>  
>  #else
> -	/* create a stack and brk area big enough for everyone
> -	 * - the brk heap starts at the bottom and works up
> -	 * - the stack starts at the top and works down
> -	 */
> +	/* create a stack area and zero-size brk area */
>  	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
>  	if (stack_size < PAGE_SIZE * 2)
>  		stack_size = PAGE_SIZE * 2;
> @@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
>  
>  	current->mm->brk = current->mm->start_brk;
>  	current->mm->context.end_brk = current->mm->start_brk;
> -	current->mm->context.end_brk +=
> -		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
>  	current->mm->start_stack = current->mm->start_brk + stack_size;
>  #endif
>  
> 

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Rich Felker Aug. 26, 2015, 2:16 a.m. UTC | #2

On Wed, Aug 26, 2015 at 11:26:02AM +1000, Greg Ungerer wrote:
> Hi Rich,
> 
> On 21/08/15 05:11, Rich Felker wrote:
> > From: Rich Felker <[e-mail]>
> > 
> > On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
> > overlap with all but the last PAGE_SIZE bytes of the stack. This leads
> > to catastrophic memory reuse/corruption if brk is used. Fix by setting
> > the brk area to zero size to disable its use.
> > 
> > Signed-off-by: Rich Felker <[e-mail]>
> 
> It would make sense to run this by David Howells <[e-mail]>,
> I think he wrote this code (added to CC list).

Thanks. I have another follow-up patch to submit soon that uses the
existing ELF_FDPIC_FLAG_CONSTDISP code in binfmt_elf_fdpic.c to load
normal, non-FDPIC ELF files on NOMMU, so I'll make sure to CC him on
that too.

> I have no problem with it, so:
> 
> Acked-by: Greg Ungerer <[e-mail]>
> 
> > ---
> > 
> > There is no reason for the kernel to be providing a brk area at all on
> > NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
> > NOMMU targets, and musl libc goes out of its way to avoid using brk
> > that might run into the stack.
> 
> I recall a long time back someone was playing with the idea of setting
> the brk to the unused parts of the last data area page. (Somewhat like
> this code seems to be trying). That scheme still allocated the full
> requested stack size (IIRC) though. And that would have been on bFLT
> executables. Anyway, just some historical reference, not really
> relevant now.

For what it's worth, musl's malloc rounds the initial brk up to a page
boundary, but the dynamic linker recovers any partial page at the end
of the data segment and donates it to malloc without brk's help. So
even though brk will fail and malloc will fall back to mmap, this
otherwise-wasted space does get recovered and used.

Rich
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David Howells Sept. 10, 2015, 2:29 p.m. UTC | #3

Greg Ungerer <[e-mail]> wrote:

> It would make sense to run this by David Howells <[e-mail]>,
> I think he wrote this code (added to CC list).
> 
> I have no problem with it, so:

Fine by me too.

Acked-by: David Howells <[e-mail]>
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Greg Ungerer Sept. 14, 2015, 12:13 p.m. UTC | #4

Hi Rich,


On 26/08/15 11:26, Greg Ungerer wrote:
> On 21/08/15 05:11, Rich Felker wrote:
>> From: Rich Felker <[e-mail]>
>>
>> On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
>> overlap with all but the last PAGE_SIZE bytes of the stack. This leads
>> to catastrophic memory reuse/corruption if brk is used. Fix by setting
>> the brk area to zero size to disable its use.
>>
>> Signed-off-by: Rich Felker <[e-mail]>
>
> It would make sense to run this by David Howells <[e-mail]>,
> I think he wrote this code (added to CC list).
>
> I have no problem with it, so:
>
> Acked-by: Greg Ungerer <[e-mail]>

Has anybody picked this up to push to Linus?
If not I can take it via the m68knommu tree.

Regards
Greg



>
>> ---
>>
>> There is no reason for the kernel to be providing a brk area at all on
>> NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
>> NOMMU targets, and musl libc goes out of its way to avoid using brk
>> that might run into the stack.
>
> I recall a long time back someone was playing with the idea of setting
> the brk to the unused parts of the last data area page. (Somewhat like
> this code seems to be trying). That scheme still allocated the full
> requested stack size (IIRC) though. And that would have been on bFLT
> executables. Anyway, just some historical reference, not really
> relevant now.
>
> Regards
> Greg
>
>
>
>> --- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
>> +++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
>> @@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
>>   		PAGE_ALIGN(current->mm->start_brk);
>>
>>   #else
>> -	/* create a stack and brk area big enough for everyone
>> -	 * - the brk heap starts at the bottom and works up
>> -	 * - the stack starts at the top and works down
>> -	 */
>> +	/* create a stack area and zero-size brk area */
>>   	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
>>   	if (stack_size < PAGE_SIZE * 2)
>>   		stack_size = PAGE_SIZE * 2;
>> @@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
>>
>>   	current->mm->brk = current->mm->start_brk;
>>   	current->mm->context.end_brk = current->mm->start_brk;
>> -	current->mm->context.end_brk +=
>> -		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
>>   	current->mm->start_stack = current->mm->start_brk + stack_size;
>>   #endif
>>
>>
>

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Rich Felker Sept. 14, 2015, 3:17 p.m. UTC | #5

On Mon, Sep 14, 2015 at 10:13:03PM +1000, Greg Ungerer wrote:
> Hi Rich,
> 
> 
> On 26/08/15 11:26, Greg Ungerer wrote:
> >On 21/08/15 05:11, Rich Felker wrote:
> >>From: Rich Felker <[e-mail]>
> >>
> >>On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
> >>overlap with all but the last PAGE_SIZE bytes of the stack. This leads
> >>to catastrophic memory reuse/corruption if brk is used. Fix by setting
> >>the brk area to zero size to disable its use.
> >>
> >>Signed-off-by: Rich Felker <[e-mail]>
> >
> >It would make sense to run this by David Howells <[e-mail]>,
> >I think he wrote this code (added to CC list).
> >
> >I have no problem with it, so:
> >
> >Acked-by: Greg Ungerer <[e-mail]>
> 
> Has anybody picked this up to push to Linus?
> If not I can take it via the m68knommu tree.

As far as I know, no. If you can do it that would be great.

Rich


> >>---
> >>
> >>There is no reason for the kernel to be providing a brk area at all on
> >>NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
> >>NOMMU targets, and musl libc goes out of its way to avoid using brk
> >>that might run into the stack.
> >
> >I recall a long time back someone was playing with the idea of setting
> >the brk to the unused parts of the last data area page. (Somewhat like
> >this code seems to be trying). That scheme still allocated the full
> >requested stack size (IIRC) though. And that would have been on bFLT
> >executables. Anyway, just some historical reference, not really
> >relevant now.
> >
> >Regards
> >Greg
> >
> >
> >
> >>--- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
> >>+++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
> >>@@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
> >>  		PAGE_ALIGN(current->mm->start_brk);
> >>
> >>  #else
> >>-	/* create a stack and brk area big enough for everyone
> >>-	 * - the brk heap starts at the bottom and works up
> >>-	 * - the stack starts at the top and works down
> >>-	 */
> >>+	/* create a stack area and zero-size brk area */
> >>  	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
> >>  	if (stack_size < PAGE_SIZE * 2)
> >>  		stack_size = PAGE_SIZE * 2;
> >>@@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
> >>
> >>  	current->mm->brk = current->mm->start_brk;
> >>  	current->mm->context.end_brk = current->mm->start_brk;
> >>-	current->mm->context.end_brk +=
> >>-		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
> >>  	current->mm->start_stack = current->mm->start_brk + stack_size;
> >>  #endif
> >>
> >>
> >
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Greg Ungerer Sept. 15, 2015, 2:13 a.m. UTC | #6

Hi Rich,

On 15/09/15 01:17, Rich Felker wrote:
> On Mon, Sep 14, 2015 at 10:13:03PM +1000, Greg Ungerer wrote:
>> On 26/08/15 11:26, Greg Ungerer wrote:
>>> On 21/08/15 05:11, Rich Felker wrote:
>>>> From: Rich Felker <[e-mail]>
>>>>
>>>> On NOMMU archs, the FDPIC ELF loader sets up the usable brk range to
>>>> overlap with all but the last PAGE_SIZE bytes of the stack. This leads
>>>> to catastrophic memory reuse/corruption if brk is used. Fix by setting
>>>> the brk area to zero size to disable its use.
>>>>
>>>> Signed-off-by: Rich Felker <[e-mail]>
>>>
>>> It would make sense to run this by David Howells <[e-mail]>,
>>> I think he wrote this code (added to CC list).
>>>
>>> I have no problem with it, so:
>>>
>>> Acked-by: Greg Ungerer <[e-mail]>
>>
>> Has anybody picked this up to push to Linus?
>> If not I can take it via the m68knommu tree.
> 
> As far as I know, no. If you can do it that would be great.

Patch applied to m68knommu git tree (for-next branch).
(https://git.kernel.org/cgit/linux/kernel/git/gerg/m68knommu.git/)

Regards
Greg


>>>> ---
>>>>
>>>> There is no reason for the kernel to be providing a brk area at all on
>>>> NOMMU; the bFLT loader does not provide one, uClibc never uses brk on
>>>> NOMMU targets, and musl libc goes out of its way to avoid using brk
>>>> that might run into the stack.
>>>
>>> I recall a long time back someone was playing with the idea of setting
>>> the brk to the unused parts of the last data area page. (Somewhat like
>>> this code seems to be trying). That scheme still allocated the full
>>> requested stack size (IIRC) though. And that would have been on bFLT
>>> executables. Anyway, just some historical reference, not really
>>> relevant now.
>>>
>>> Regards
>>> Greg
>>>
>>>
>>>
>>>> --- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
>>>> +++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
>>>> @@ -374,10 +388,7 @@ static int load_elf_fdpic_binary(struct
>>>>  		PAGE_ALIGN(current->mm->start_brk);
>>>>
>>>>  #else
>>>> -	/* create a stack and brk area big enough for everyone
>>>> -	 * - the brk heap starts at the bottom and works up
>>>> -	 * - the stack starts at the top and works down
>>>> -	 */
>>>> +	/* create a stack area and zero-size brk area */
>>>>  	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
>>>>  	if (stack_size < PAGE_SIZE * 2)
>>>>  		stack_size = PAGE_SIZE * 2;
>>>> @@ -400,8 +411,6 @@ static int load_elf_fdpic_binary(struct
>>>>
>>>>  	current->mm->brk = current->mm->start_brk;
>>>>  	current->mm->context.end_brk = current->mm->start_brk;
>>>> -	current->mm->context.end_brk +=
>>>> -		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
>>>>  	current->mm->start_stack = current->mm->start_brk + stack_size;
>>>>  #endif
>>>>
>>>>
>>>
> 

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Patch

--- fs/binfmt_elf_fdpic.c.orig	2015-08-20 18:05:19.089888654 +0000
+++ fs/binfmt_elf_fdpic.c	2015-08-20 18:10:01.519871432 +0000
@@ -374,10 +388,7 @@  static int load_elf_fdpic_binary(struct
 		PAGE_ALIGN(current->mm->start_brk);
 
 #else
-	/* create a stack and brk area big enough for everyone
-	 * - the brk heap starts at the bottom and works up
-	 * - the stack starts at the top and works down
-	 */
+	/* create a stack area and zero-size brk area */
 	stack_size = (stack_size + PAGE_SIZE - 1) & PAGE_MASK;
 	if (stack_size < PAGE_SIZE * 2)
 		stack_size = PAGE_SIZE * 2;
@@ -400,8 +411,6 @@  static int load_elf_fdpic_binary(struct
 
 	current->mm->brk = current->mm->start_brk;
 	current->mm->context.end_brk = current->mm->start_brk;
-	current->mm->context.end_brk +=
-		(stack_size > PAGE_SIZE) ? (stack_size - PAGE_SIZE) : 0;
 	current->mm->start_stack = current->mm->start_brk + stack_size;
 #endif