Re: [PATCH 01/18] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

[Vlastimil Babka <vbabka@suse.cz>]

On 11/01/2017 04:36 PM, Jan Kara wrote:
> From: Dan Williams <dan.j.williams@intel.com>
> 
> The mmap(2) syscall suffers from the ABI anti-pattern of not validating
> unknown flags. However, proposals like MAP_SYNC need a mechanism to
> define new behavior that is known to fail on older kernels without the
> support. Define a new MAP_SHARED_VALIDATE flag pattern that is
> guaranteed to fail on all legacy mmap implementations.

So I'm trying to make sense of this together with Michal's attempt for
MAP_FIXED_SAFE [1] where he has to introduce a completely new flag
instead of flag modifier exactly for the reason of not validating
unknown flags. And my conclusion is that because MAP_SHARED_VALIDATE
implies MAP_SHARED and excludes MAP_PRIVATE, MAP_FIXED_SAFE as a
modifier cannot build on top of this. Wouldn't thus it be really better
long-term to introduce mmap3 at this point? ...

Vlastimil

[1] https://marc.info/?l=linux-arch&m=151083448322336&w=2

> It is worth noting that the original proposal was for a standalone
> MAP_VALIDATE flag. However, when that  could not be supported by all
> archs Linus observed:
> 
>     I see why you *think* you want a bitmap. You think you want
>     a bitmap because you want to make MAP_VALIDATE be part of MAP_SYNC
>     etc, so that people can do
> 
>     ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED
> 		    | MAP_SYNC, fd, 0);
> 
>     and "know" that MAP_SYNC actually takes.
> 
>     And I'm saying that whole wish is bogus. You're fundamentally
>     depending on special semantics, just make it explicit. It's already
>     not portable, so don't try to make it so.
> 
>     Rename that MAP_VALIDATE as MAP_SHARED_VALIDATE, make it have a value
>     of 0x3, and make people do
> 
>     ret = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED_VALIDATE
> 		    | MAP_SYNC, fd, 0);
> 
>     and then the kernel side is easier too (none of that random garbage
>     playing games with looking at the "MAP_VALIDATE bit", but just another
>     case statement in that map type thing.
> 
>     Boom. Done.
> 
> Similar to ->fallocate() we also want the ability to validate the
> support for new flags on a per ->mmap() 'struct file_operations'
> instance basis.  Towards that end arrange for flags to be generically
> validated against a mmap_supported_flags exported by 'struct
> file_operations'. By default all existing flags are implicitly
> supported, but new flags require MAP_SHARED_VALIDATE and
> per-instance-opt-in.
> 
> Cc: Jan Kara <jack@suse.cz>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Suggested-by: Christoph Hellwig <hch@lst.de>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  arch/alpha/include/uapi/asm/mman.h           |  1 +
>  arch/mips/include/uapi/asm/mman.h            |  1 +
>  arch/parisc/include/uapi/asm/mman.h          |  1 +
>  arch/xtensa/include/uapi/asm/mman.h          |  1 +
>  include/linux/fs.h                           |  1 +
>  include/linux/mman.h                         | 39 ++++++++++++++++++++++++++++
>  include/uapi/asm-generic/mman-common.h       |  1 +
>  mm/mmap.c                                    | 15 +++++++++++
>  tools/include/uapi/asm-generic/mman-common.h |  1 +
>  9 files changed, 61 insertions(+)
> 
> diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
> index 3b26cc62dadb..f6d118aaedb9 100644
> --- a/arch/alpha/include/uapi/asm/mman.h
> +++ b/arch/alpha/include/uapi/asm/mman.h
> @@ -11,6 +11,7 @@
>  
>  #define MAP_SHARED	0x01		/* Share changes */
>  #define MAP_PRIVATE	0x02		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
>  #define MAP_TYPE	0x0f		/* Mask for type of mapping (OSF/1 is _wrong_) */
>  #define MAP_FIXED	0x100		/* Interpret addr exactly */
>  #define MAP_ANONYMOUS	0x10		/* don't use a file */
> diff --git a/arch/mips/include/uapi/asm/mman.h b/arch/mips/include/uapi/asm/mman.h
> index da3216007fe0..93268e4cd3c7 100644
> --- a/arch/mips/include/uapi/asm/mman.h
> +++ b/arch/mips/include/uapi/asm/mman.h
> @@ -28,6 +28,7 @@
>   */
>  #define MAP_SHARED	0x001		/* Share changes */
>  #define MAP_PRIVATE	0x002		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x003	/* share + validate extension flags */
>  #define MAP_TYPE	0x00f		/* Mask for type of mapping */
>  #define MAP_FIXED	0x010		/* Interpret addr exactly */
>  
> diff --git a/arch/parisc/include/uapi/asm/mman.h b/arch/parisc/include/uapi/asm/mman.h
> index 775b5d5e41a1..bca652aa1677 100644
> --- a/arch/parisc/include/uapi/asm/mman.h
> +++ b/arch/parisc/include/uapi/asm/mman.h
> @@ -11,6 +11,7 @@
>  
>  #define MAP_SHARED	0x01		/* Share changes */
>  #define MAP_PRIVATE	0x02		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
>  #define MAP_TYPE	0x03		/* Mask for type of mapping */
>  #define MAP_FIXED	0x04		/* Interpret addr exactly */
>  #define MAP_ANONYMOUS	0x10		/* don't use a file */
> diff --git a/arch/xtensa/include/uapi/asm/mman.h b/arch/xtensa/include/uapi/asm/mman.h
> index b15b278aa314..9ab426374714 100644
> --- a/arch/xtensa/include/uapi/asm/mman.h
> +++ b/arch/xtensa/include/uapi/asm/mman.h
> @@ -35,6 +35,7 @@
>   */
>  #define MAP_SHARED	0x001		/* Share changes */
>  #define MAP_PRIVATE	0x002		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x003	/* share + validate extension flags */
>  #define MAP_TYPE	0x00f		/* Mask for type of mapping */
>  #define MAP_FIXED	0x010		/* Interpret addr exactly */
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 13dab191a23e..57added3201d 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1701,6 +1701,7 @@ struct file_operations {
>  	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
>  	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
>  	int (*mmap) (struct file *, struct vm_area_struct *);
> +	unsigned long mmap_supported_flags;
>  	int (*open) (struct inode *, struct file *);
>  	int (*flush) (struct file *, fl_owner_t id);
>  	int (*release) (struct inode *, struct file *);
> diff --git a/include/linux/mman.h b/include/linux/mman.h
> index c8367041fafd..94b63b4d71ff 100644
> --- a/include/linux/mman.h
> +++ b/include/linux/mman.h
> @@ -7,6 +7,45 @@
>  #include <linux/atomic.h>
>  #include <uapi/linux/mman.h>
>  
> +/*
> + * Arrange for legacy / undefined architecture specific flags to be
> + * ignored by default in LEGACY_MAP_MASK.
> + */
> +#ifndef MAP_32BIT
> +#define MAP_32BIT 0
> +#endif
> +#ifndef MAP_HUGE_2MB
> +#define MAP_HUGE_2MB 0
> +#endif
> +#ifndef MAP_HUGE_1GB
> +#define MAP_HUGE_1GB 0
> +#endif
> +#ifndef MAP_UNINITIALIZED
> +#define MAP_UNINITIALIZED 0
> +#endif
> +
> +/*
> + * The historical set of flags that all mmap implementations implicitly
> + * support when a ->mmap_validate() op is not provided in file_operations.
> + */
> +#define LEGACY_MAP_MASK (MAP_SHARED \
> +		| MAP_PRIVATE \
> +		| MAP_FIXED \
> +		| MAP_ANONYMOUS \
> +		| MAP_DENYWRITE \
> +		| MAP_EXECUTABLE \
> +		| MAP_UNINITIALIZED \
> +		| MAP_GROWSDOWN \
> +		| MAP_LOCKED \
> +		| MAP_NORESERVE \
> +		| MAP_POPULATE \
> +		| MAP_NONBLOCK \
> +		| MAP_STACK \
> +		| MAP_HUGETLB \
> +		| MAP_32BIT \
> +		| MAP_HUGE_2MB \
> +		| MAP_HUGE_1GB)
> +
>  extern int sysctl_overcommit_memory;
>  extern int sysctl_overcommit_ratio;
>  extern unsigned long sysctl_overcommit_kbytes;
> diff --git a/include/uapi/asm-generic/mman-common.h b/include/uapi/asm-generic/mman-common.h
> index 203268f9231e..8ce7f5a0800f 100644
> --- a/include/uapi/asm-generic/mman-common.h
> +++ b/include/uapi/asm-generic/mman-common.h
> @@ -16,6 +16,7 @@
>  
>  #define MAP_SHARED	0x01		/* Share changes */
>  #define MAP_PRIVATE	0x02		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
>  #define MAP_TYPE	0x0f		/* Mask for type of mapping */
>  #define MAP_FIXED	0x10		/* Interpret addr exactly */
>  #define MAP_ANONYMOUS	0x20		/* don't use a file */
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 680506faceae..924839fac0e6 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1387,9 +1387,24 @@ unsigned long do_mmap(struct file *file, unsigned long addr,
>  
>  	if (file) {
>  		struct inode *inode = file_inode(file);
> +		unsigned long flags_mask;
> +
> +		flags_mask = LEGACY_MAP_MASK | file->f_op->mmap_supported_flags;
>  
>  		switch (flags & MAP_TYPE) {
>  		case MAP_SHARED:
> +			/*
> +			 * Force use of MAP_SHARED_VALIDATE with non-legacy
> +			 * flags. E.g. MAP_SYNC is dangerous to use with
> +			 * MAP_SHARED as you don't know which consistency model
> +			 * you will get. We silently ignore unsupported flags
> +			 * with MAP_SHARED to preserve backward compatibility.
> +			 */
> +			flags &= LEGACY_MAP_MASK;
> +			/* fall through */
> +		case MAP_SHARED_VALIDATE:
> +			if (flags & ~flags_mask)
> +				return -EOPNOTSUPP;
>  			if ((prot&PROT_WRITE) && !(file->f_mode&FMODE_WRITE))
>  				return -EACCES;
>  
> diff --git a/tools/include/uapi/asm-generic/mman-common.h b/tools/include/uapi/asm-generic/mman-common.h
> index 203268f9231e..8ce7f5a0800f 100644
> --- a/tools/include/uapi/asm-generic/mman-common.h
> +++ b/tools/include/uapi/asm-generic/mman-common.h
> @@ -16,6 +16,7 @@
>  
>  #define MAP_SHARED	0x01		/* Share changes */
>  #define MAP_PRIVATE	0x02		/* Changes are private */
> +#define MAP_SHARED_VALIDATE 0x03	/* share + validate extension flags */
>  #define MAP_TYPE	0x0f		/* Mask for type of mapping */
>  #define MAP_FIXED	0x10		/* Interpret addr exactly */
>  #define MAP_ANONYMOUS	0x20		/* don't use a file */
> 

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