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SubjectRe: [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@suse.com>
>
> 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@suse.com>
> ---
> 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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