Re: [PATCH] mm: fix incorrect behavior when process virtual address space limit is exceeded

[Michal Hocko <mhocko@kernel.org>]

[CCing Cyril]

On Mon 16-11-15 18:36:19, Piotr Kwapulinski wrote:
> When a new virtual memory area is added to the process's virtual address
> space and this vma causes the process's virtual address space limit
> (RLIMIT_AS) to be exceeded then kernel behaves incorrectly. Incorrect
> behavior is a result of a kernel bug. The kernel in most cases
> unnecessarily scans the entire process's virtual address space trying to
> find the overlapping vma with the virtual memory region being added.
> The kernel incorrectly compares the MAP_FIXED flag with vm_flags variable
> in mmap_region function. The vm_flags variable should not be compared
> with MAP_FIXED flag. The MAP_FIXED flag has got the same numerical value
> as VM_MAYREAD flag (0x10). As a result the following test
> from mmap_region:
> 
> if (!(vm_flags & MAP_FIXED))
> is in fact:
> if (!(vm_flags & VM_MAYREAD))

It seems this has been broken since it was introduced e8420a8ece80
("mm/mmap: check for RLIMIT_AS before unmapping"). The patch has fixed
the case where unmap happened before the we tested may_expand_vm and
failed which was sufficient for the LTP test case but it apparently
never tried to consider the overlapping ranges to eventually allow to
create the mapping.

> The VM_MAYREAD flag is almost always set in vm_flags while MAP_FIXED
> flag is not so common. The result of the above condition is somewhat
> reverted.
> This patch fixes this bug. It causes that the kernel tries to find the
> overlapping vma only when the requested virtual memory region has got
> the fixed starting virtual address (MAP_FIXED flag set).
> For tile architecture Calling mmap_region with the MAP_FIXED flag only is
> sufficient. However the MAP_ANONYMOUS and MAP_PRIVATE flags are passed for
> the completeness of the solution.

I found the changelog rather hard to grasp. But the fix is correct. The
user visible effect is that RLIMIT_AS might hit for a MAP_FIXED
mapping even though it wouldn't enlarge the used address space.

> Signed-off-by: Piotr Kwapulinski <kwapulinski.piotr@gmail.com>

Acked-by: Michal Hocko <mhocko@suse.com>

but I would encourage you to rephrase the changelog. Something like the
following maybe? Also I would rename flags to mmap_flags to be more
clear.
"
e8420a8ece80 ("mm/mmap: check for RLIMIT_AS before unmapping") has tried
to make MAP_FIXED bahavior wrt RLIMIT_AS more graceful. One of the issue
was that the original mapping shouldn't be destroyed if the mmap call
had to fail due to the address limit. Another one was that MAP_FIXED is
destroying an existing mapping and so might not enlarge the used address
space in the end and then the mmap should suceed. The later case is
implemented by checking count_vma_pages_range but it never really worked
because MAP_FIXED flag is compared against vm_flags rather than mmap
flags. This means that the overlapping areas were never checked because
MAP_FIXED has the same value as VM_MAYREAD which is basically always set.

Fix the issue by introducing a new parameter to mmap_region which forwards
the mmap flags and now the MAP_FIXED can be checked properly.
Tile and its arch_setup_additional_pages as the only in tree user of
mmap_region has to be specific about its mmap flags now.
"

I guess this is also a stable material.

> ---
>  arch/tile/mm/elf.c | 1 +
>  include/linux/mm.h | 3 ++-
>  mm/mmap.c          | 7 ++++---
>  3 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/tile/mm/elf.c b/arch/tile/mm/elf.c
> index 6225cc9..dae4b33 100644
> --- a/arch/tile/mm/elf.c
> +++ b/arch/tile/mm/elf.c
> @@ -142,6 +142,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm,
>  	if (!retval) {
>  		unsigned long addr = MEM_USER_INTRPT;
>  		addr = mmap_region(NULL, addr, INTRPT_SIZE,
> +				   MAP_FIXED|MAP_ANONYMOUS|MAP_PRIVATE,
>  				   VM_READ|VM_EXEC|
>  				   VM_MAYREAD|VM_MAYWRITE|VM_MAYEXEC, 0);
>  		if (addr > (unsigned long) -PAGE_SIZE)
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 00bad77..1ae21c1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1911,7 +1911,8 @@ extern int install_special_mapping(struct mm_struct *mm,
>  extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
>  
>  extern unsigned long mmap_region(struct file *file, unsigned long addr,
> -	unsigned long len, vm_flags_t vm_flags, unsigned long pgoff);
> +	unsigned long len, unsigned long flags,
> +	vm_flags_t vm_flags, unsigned long pgoff);
>  extern unsigned long do_mmap(struct file *file, unsigned long addr,
>  	unsigned long len, unsigned long prot, unsigned long flags,
>  	vm_flags_t vm_flags, unsigned long pgoff, unsigned long *populate);
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2ce04a6..ad8b845 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1399,7 +1399,7 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  			vm_flags |= VM_NORESERVE;
>  	}
>  
> -	addr = mmap_region(file, addr, len, vm_flags, pgoff);
> +	addr = mmap_region(file, addr, len, flags, vm_flags, pgoff);
>  	if (!IS_ERR_VALUE(addr) &&
>  	    ((vm_flags & VM_LOCKED) ||
>  	     (flags & (MAP_POPULATE | MAP_NONBLOCK)) == MAP_POPULATE))
> @@ -1535,7 +1535,8 @@ static inline int accountable_mapping(struct file *file, vm_flags_t vm_flags)
>  }
>  
>  unsigned long mmap_region(struct file *file, unsigned long addr,
> -		unsigned long len, vm_flags_t vm_flags, unsigned long pgoff)
> +		unsigned long len, unsigned long flags,
> +		vm_flags_t vm_flags, unsigned long pgoff)
>  {
>  	struct mm_struct *mm = current->mm;
>  	struct vm_area_struct *vma, *prev;
> @@ -1551,7 +1552,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		 * MAP_FIXED may remove pages of mappings that intersects with
>  		 * requested mapping. Account for the pages it would unmap.
>  		 */
> -		if (!(vm_flags & MAP_FIXED))
> +		if (!(flags & MAP_FIXED))
>  			return -ENOMEM;
>  
>  		nr_pages = count_vma_pages_range(mm, addr, addr + len);
> -- 
> 2.6.2
> 

-- 
Michal Hocko
SUSE Labs

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