Re: mm: insure topdown mmap chooses addresses above security minimum

* Timothy Pepper [off-list ref] wrote:

A security check is performed on mmap addresses in
security/security.c:security_mmap_addr().  It uses mmap_min_addr to insure
mmaps don't get addresses lower than a user configurable guard value
(/proc/sys/vm/mmap_min_addr).  The arch specific mmap topdown searches
look for a map candidate address all the way down to a low_limit that is
currently hard coded as PAGE_SIZE.  Depending on compile time options
and userspace setting the procfs tunable, the security check's view of
the minimum allowable address may be something greater than PAGE_SIZE.
This leaves a gap where get_unmapped_area()'s call to get_area() might
return an address above PAGE_SIZE, but below mmap_min_addr, and thus
get_unmapped_area() fails.

This was seen on x86_64 in the case of a topdown address space and a large
stack rlimit, with mmap_min_addr having been set to 32k by the distro.
This left a 28k gap where the get area search intends to place a small
mmap, but then get_unmapped_area() stumbles at the security check.

What should have happened is the address search wraps back to a higher
address, the search continues and perhaps succeeds.  Indeed an mmap of
a larger size gets a topdown search that does wrap around back up into
the rlimit stack reserve and succeeds assuming suitable free space.
But a small mmap fits in the low gap and always fails.  It becomes
possible to make large mmaps but not small ones.

When an explicit address hint is given, mm/mmap.c's round_hint_to_min()
will round up to mmap_min_addr.

A topdown search's low_limit should similarly consider mmap_min_addr
instead of just PAGE_SIZE.

Signed-off-by: Tim Pepper <redacted>
Cc: <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Russell King <redacted>
Cc: <redacted>
Cc: Ralf Baechle <redacted>
Cc: <redacted>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <redacted>
Cc: <redacted>
Cc: Paul Mundt <redacted>
Cc: <redacted>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: <redacted>
--
 arch/arm/mm/mmap.c               | 3 ++-
 arch/mips/mm/mmap.c              | 3 ++-
 arch/powerpc/mm/slice.c          | 3 ++-
 arch/sh/mm/mmap.c                | 3 ++-
 arch/sparc/kernel/sys_sparc_64.c | 3 ++-
 arch/x86/kernel/sys_x86_64.c     | 3 ++-
 6 files changed, 12 insertions(+), 6 deletions(-)

+	info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr));
 	info.high_limit = mm->mmap_base;
 	info.align_mask = do_align ? (PAGE_MASK & (SHMLBA - 1)) : 0;
 	info.align_offset = pgoff << PAGE_SHIFT;

-		info.low_limit = addr;
+		info.low_limit = max(addr, PAGE_ALIGN(mmap_min_addr));

There appears to be a lot of repetition in these methods - instead of 
changing 6 places it would be more future-proof to first factor out the 
common bits and then to apply the fix to the shared implementation.

Thanks,

	Ingo