Re: [PATCH v8 01/14] mm: introduce MAP_SHARED_VALIDATE, a mechanism to safely define new mmap flags

On Wed, Oct 11, 2017 at 12:43 AM, Jan Kara [off-list ref] wrote:

On Tue 10-10-17 07:49:01, Dan Williams wrote:

quoted

The mmap(2) syscall suffers from the ABI anti-pattern of not validating
unknown flags. However, proposals like MAP_SYNC and MAP_DIRECT 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.

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_mask 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 <redacted>
Cc: Arnd Bergmann <redacted>
Cc: Andy Lutomirski <redacted>
Cc: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Suggested-by: Christoph Hellwig <redacted>
Suggested-by: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>
Signed-off-by: Dan Williams <redacted>
---
 arch/alpha/include/uapi/asm/mman.h           |    1 +
 arch/mips/include/uapi/asm/mman.h            |    1 +
 arch/mips/kernel/vdso.c                      |    2 +
 arch/parisc/include/uapi/asm/mman.h          |    1 +
 arch/tile/mm/elf.c                           |    3 +-
 arch/xtensa/include/uapi/asm/mman.h          |    1 +
 include/linux/fs.h                           |    2 +
 include/linux/mm.h                           |    2 +
 include/linux/mman.h                         |   39 ++++++++++++++++++++++++++
 include/uapi/asm-generic/mman-common.h       |    1 +
 mm/mmap.c                                    |   21 ++++++++++++--
 tools/include/uapi/asm-generic/mman-common.h |    1 +
 12 files changed, 69 insertions(+), 6 deletions(-)

diff --git a/arch/alpha/include/uapi/asm/mman.h b/arch/alpha/include/uapi/asm/mman.h
index 3b26cc62dadb..92823f24890b 100644
--- a/arch/alpha/include/uapi/asm/mman.h
+++ b/arch/alpha/include/uapi/asm/mman.h

@@ -14,6 +14,7 @@
 #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 */
+#define MAP_SHARED_VALIDATE 0x3              /* share + validate extension flags */

Just a nit but I'd put definition of MAP_SHARED_VALIDATE close to the
definition of MAP_SHARED and MAP_PRIVATE where it logically belongs (for
all archs).

Will do.

quoted

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f8c10d336e42..5c4c98e4adc9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h

@@ -2133,7 +2133,7 @@ extern unsigned long get_unmapped_area(struct file *, unsigned long, unsigned lo

 extern unsigned long mmap_region(struct file *file, unsigned long addr,
      unsigned long len, vm_flags_t vm_flags, unsigned long pgoff,
-     struct list_head *uf);
+     struct list_head *uf, unsigned long map_flags);
 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,

I have to say I'm not very keen on passing down both vm_flags and map_flags
- vm_flags are almost a subset of map_flags but not quite and the ambiguity
which needs to be used for a particular check seems to open a space for
errors. Granted you currently only care about MAP_DIRECT in ->mmap_validate
and just pass map_flags through mmap_region() so there's no space for
confusion but future checks could do something different.

I was hoping the fact that one can't trigger a call to
->mmap_validate() unless they specify a flag outside of
LEGACY_MAP_MASK makes it clearer that validation is only for new
flags. Old flags get the existing "may be silently ignored" behavior.

But OTOH I don't
see a cleaner way of avoiding the need to allocate vma flag for something
you need to check down in ->mmap_validate so I guess I'll live with that
and if problems really happen, we may have cleaner idea what needs to be
done.

So overall feel free to add:

Reviewed-by: Jan Kara <redacted>

Thanks Jan.