* Timothy Pepper <timothy.c.pepper@xxxxxxxxxxxxxxx> 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 <timothy.c.pepper@xxxxxxxxxxxxxxx> > Cc: linux-mm@xxxxxxxxx > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > Cc: x86@xxxxxxxxxx > Cc: Russell King <linux@xxxxxxxxxxxxxxxx> > Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx > Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx> > Cc: linux-mips@xxxxxxxxxxxxxx > Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> > Cc: Paul Mackerras <paulus@xxxxxxxxx> > Cc: linuxppc-dev@xxxxxxxxxxxxxxxx > Cc: Paul Mundt <lethal@xxxxxxxxxxxx> > Cc: linux-sh@xxxxxxxxxxxxxxx > Cc: "David S. Miller" <davem@xxxxxxxxxxxxx> > Cc: sparclinux@xxxxxxxxxxxxxxx > -- > 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.flags = VM_UNMAPPED_AREA_TOPDOWN; > - info.low_limit = PAGE_SIZE; > + info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr)); > info.high_limit = mm->mmap_base; > addr = vm_unmapped_area(&info); > - info.low_limit = addr; > + info.low_limit = max(addr, PAGE_ALIGN(mmap_min_addr)); > info.flags = VM_UNMAPPED_AREA_TOPDOWN; > info.length = len; > - info.low_limit = PAGE_SIZE; > + info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr)); > info.high_limit = mm->mmap_base; > info.align_mask = do_colour_align ? (PAGE_MASK & shm_align_mask) : 0; > info.align_offset = pgoff << PAGE_SHIFT; > info.flags = VM_UNMAPPED_AREA_TOPDOWN; > info.length = len; > - info.low_limit = PAGE_SIZE; > + info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr)); > info.high_limit = mm->mmap_base; > info.align_mask = do_color_align ? (PAGE_MASK & (SHMLBA - 1)) : 0; > info.align_offset = pgoff << PAGE_SHIFT; > info.flags = VM_UNMAPPED_AREA_TOPDOWN; > info.length = len; > - info.low_limit = PAGE_SIZE; > + info.low_limit = max(PAGE_SIZE, PAGE_ALIGN(mmap_min_addr)); > info.high_limit = mm->mmap_base; > info.align_mask = filp ? get_align_mask() : 0; > info.align_offset = pgoff << PAGE_SHIFT; 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
