Re: [RFC PATCH 1/2] mm: introduce MAP_FIXED_SAFE

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On 11/16/2017 02:18 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> MAP_FIXED is used quite often to enforce mapping at the particular
> range. The main problem of this flag is, however, that it is inherently
> dangerous because it unmaps existing mappings covered by the requested
> range. This can cause silent memory corruptions. Some of them even with
> serious security implications. While the current semantic might be
> really desiderable in many cases there are others which would want to
> enforce the given range but rather see a failure than a silent memory
> corruption on a clashing range. Please note that there is no guarantee
> that a given range is obeyed by the mmap even when it is free - e.g.
> arch specific code is allowed to apply an alignment.
> 
> Introduce a new MAP_FIXED_SAFE flag for mmap to achieve this behavior.
> It has the same semantic as MAP_FIXED wrt. the given address request
> with a single exception that it fails with ENOMEM if the requested
> address is already covered by an existing mapping. We still do rely on
> get_unmaped_area to handle all the arch specific MAP_FIXED treatment and
> check for a conflicting vma after it returns.
> 
> [set MAP_FIXED before round_hint_to_min as per Khalid Aziz]
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  arch/alpha/include/uapi/asm/mman.h   |  2 ++
>  arch/mips/include/uapi/asm/mman.h    |  2 ++
>  arch/parisc/include/uapi/asm/mman.h  |  2 ++
>  arch/powerpc/include/uapi/asm/mman.h |  1 +
>  arch/sparc/include/uapi/asm/mman.h   |  1 +
>  arch/tile/include/uapi/asm/mman.h    |  1 +
>  arch/xtensa/include/uapi/asm/mman.h  |  2 ++
>  include/uapi/asm-generic/mman.h      |  1 +
>  mm/mmap.c                            | 11 +++++++++++
>  9 files changed, 23 insertions(+)
> 
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 3b26cc62dadb..0e5724e4b4ad 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -31,6 +31,8 @@
>  #define MAP_STACK	0x80000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x100000	/* create a huge page mapping */
>  
> +#define MAP_FIXED_SAFE	0x200000	/* MAP_FIXED which doesn't unmap underlying mapping */
> +
>  #define MS_ASYNC	1		/* sync memory asynchronously */
>  #define MS_SYNC		2		/* synchronous memory sync */
>  #define MS_INVALIDATE	4		/* invalidate the caches */
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index da3216007fe0..fc5e61ef9fd4 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -49,6 +49,8 @@
>  #define MAP_STACK	0x40000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x80000		/* create a huge page mapping */
>  
> +#define MAP_FIXED_SAFE	0x100000	/* MAP_FIXED which doesn't unmap underlying mapping */
> +
>  /*
>   * Flags for msync
>   */
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index cc9ba1d34779..c926487472fa 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -25,6 +25,8 @@
>  #define MAP_STACK	0x40000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x80000		/* create a huge page mapping */
>  
> +#define MAP_FIXED_SAFE	0x100000	/* MAP_FIXED which doesn't unmap underlying mapping */
> +
>  #define MS_SYNC		1		/* synchronous memory sync */
>  #define MS_ASYNC	2		/* sync memory asynchronously */
>  #define MS_INVALIDATE	4		/* invalidate the caches */
> diff --git a/arch/powerpc/include/uapi/asm/mman.h b/arch/powerpc/include/uapi/asm/mman.h
> index 03c06ba7464f..d97342ca25b1 100644
> --- a/arch/powerpc/include/uapi/asm/mman.h
> +++ b/arch/powerpc/include/uapi/asm/mman.h
> @@ -28,5 +28,6 @@
>  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
>  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> +#define MAP_FIXED_SAFE	0x800000	/* MAP_FIXED which doesn't unmap underlying mapping */
 

Hi Michal,

1. The powerpc change, above, has one too many zeroes. It should be 0x80000, 
not 0x800000.

2. For the one-line comments, if you phrase them like this:

/* Like MAP_FIXED, except that it doesn't unmap pre-existing mappings */

...I think that would be a little clearer.

more below:

>  #endif /* _UAPI_ASM_POWERPC_MMAN_H */
> diff --git a/arch/sparc/include/uapi/asm/mman.h b/arch/sparc/include/uapi/asm/mman.h
> index 9765896ecb2c..7b00477a7f9a 100644
> --- a/arch/sparc/include/uapi/asm/mman.h
> +++ b/arch/sparc/include/uapi/asm/mman.h
> @@ -23,6 +23,7 @@
>  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
>  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> +#define MAP_FIXED_SAFE	0x80000		/* MAP_FIXED which doesn't unmap underlying mapping */
>  
>  
>  #endif /* _UAPI__SPARC_MMAN_H__ */
> diff --git a/arch/tile/include/uapi/asm/mman.h b/arch/tile/include/uapi/asm/mman.h
> index 63ee13faf17d..d5d58d2dc95e 100644
> --- a/arch/tile/include/uapi/asm/mman.h
> +++ b/arch/tile/include/uapi/asm/mman.h
> @@ -29,6 +29,7 @@
>  #define MAP_DENYWRITE	0x0800		/* ETXTBSY */
>  #define MAP_EXECUTABLE	0x1000		/* mark it as an executable */
>  #define MAP_HUGETLB	0x4000		/* create a huge page mapping */
> +#define MAP_FIXED_SAFE	0x8000		/* MAP_FIXED which doesn't unmap underlying mapping */
>  
>  
>  /*
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index b15b278aa314..d665bd8b7cbd 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -55,6 +55,7 @@
>  #define MAP_NONBLOCK	0x20000		/* do not block on IO */
>  #define MAP_STACK	0x40000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x80000		/* create a huge page mapping */
> +#define MAP_FIXED_SAFE	0x100000	/* MAP_FIXED which doesn't unmap underlying mapping */
>  #ifdef CONFIG_MMAP_ALLOW_UNINITIALIZED
>  # define MAP_UNINITIALIZED 0x4000000	/* For anonymous mmap, memory could be
>  					 * uninitialized */
> @@ -62,6 +63,7 @@
>  # define MAP_UNINITIALIZED 0x0		/* Don't support this flag */
>  #endif
>  
> +
>  /*
>   * Flags for msync
>   */
> diff --git a/include/uapi/asm-generic/mman.h b/include/uapi/asm-generic/mman.h
> index 7162cd4cca73..64c46047fbd3 100644
> --- a/include/uapi/asm-generic/mman.h
> +++ b/include/uapi/asm-generic/mman.h
> @@ -12,6 +12,7 @@
>  #define MAP_NONBLOCK	0x10000		/* do not block on IO */
>  #define MAP_STACK	0x20000		/* give out an address that is best suited for process/thread stacks */
>  #define MAP_HUGETLB	0x40000		/* create a huge page mapping */
> +#define MAP_FIXED_SAFE	0x80000		/* MAP_FIXED which doesn't unmap underlying mapping */
>  
>  /* Bits [26:31] are reserved, see mman-common.h for MAP_HUGETLB usage */
>  
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 680506faceae..89af0b5839a5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1342,6 +1342,10 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  		if (!(file && path_noexec(&file->f_path)))
>  			prot |= PROT_EXEC;
>  
> +	/* force arch specific MAP_FIXED handling in get_unmapped_area */
> +	if (flags & MAP_FIXED_SAFE)
> +		flags |= MAP_FIXED;

Hooking in at this point is a nice way to solve the problem. :)

For the naming and implementation, I see a couple of things that might improve
it slightly:

a) Change MAP_FIXED_SAFE to MAP_NO_CLOBBER (as per Kees' idea), but keep the
new flag independent, by omitting the above two lines. Instead of forcing
MAP_FIXED as a result of the new flag, you could simply fail to take any action
on MAP_NO_CLOBBER *unless* MAP_FIXED is set.

This is a bit easier to explain and reason about, as compared to a flag that
auto-sets another flag. I like this approach best.

  or

b) Change MAP_FIXED_SAFE to MAP_FIXED_NO_CLOBBER (also a variation on Kees' name
idea, but a little longer, a bit uglier, and clearer), and leave the implementation
the same.

thanks,
John Hubbard

> +
>  	if (!(flags & MAP_FIXED))
>  		addr = round_hint_to_min(addr);
>  
> @@ -1365,6 +1369,13 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  	if (offset_in_page(addr))
>  		return addr;
>  
> +	if (flags & MAP_FIXED_SAFE) {
> +		struct vm_area_struct *vma = find_vma(mm, addr);
> +
> +		if (vma && vma->vm_start <= addr)
> +			return -ENOMEM;
> +	}
> +
>  	if (prot == PROT_EXEC) {
>  		pkey = execute_only_pkey(mm);
>  		if (pkey < 0)
> 
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