• Hugh Dickins's avatar
    mm: larger stack guard gap, between vmas · cfc0eb40
    Hugh Dickins authored
    commit 1be7107f upstream.
    
    Stack guard page is a useful feature to reduce a risk of stack smashing
    into a different mapping. We have been using a single page gap which
    is sufficient to prevent having stack adjacent to a different mapping.
    But this seems to be insufficient in the light of the stack usage in
    userspace. E.g. glibc uses as large as 64kB alloca() in many commonly
    used functions. Others use constructs liks gid_t buffer[NGROUPS_MAX]
    which is 256kB or stack strings with MAX_ARG_STRLEN.
    
    This will become especially dangerous for suid binaries and the default
    no limit for the stack size limit because those applications can be
    tricked to consume a large portion of the stack and a single glibc call
    could jump over the guard page. These attacks are not theoretical,
    unfortunatelly.
    
    Make those attacks less probable by increasing the stack guard gap
    to 1MB (on systems with 4k pages; but make it depend on the page size
    because systems with larger base pages might cap stack allocations in
    the PAGE_SIZE units) which should cover larger alloca() and VLA stack
    allocations. It is obviously not a full fix because the problem is
    somehow inherent, but it should reduce attack space a lot.
    
    One could argue that the gap size should be configurable from userspace,
    but that can be done later when somebody finds that the new 1MB is wrong
    for some special case applications.  For now, add a kernel command line
    option (stack_guard_gap) to specify the stack gap size (in page units).
    
    Implementation wise, first delete all the old code for stack guard page:
    because although we could get away with accounting one extra page in a
    stack vma, accounting a larger gap can break userspace - case in point,
    a program run with "ulimit -S -v 20000" failed when the 1MB gap was
    counted for RLIMIT_AS; similar problems could come with RLIMIT_MLOCK
    and strict non-overcommit mode.
    
    Instead of keeping gap inside the stack vma, maintain the stack guard
    gap as a gap between vmas: using vm_start_gap() in place of vm_start
    (or vm_end_gap() in place of vm_end if VM_GROWSUP) in just those few
    places which need to respect the gap - mainly arch_get_unmapped_area(),
    and and the vma tree's subtree_gap support for that.
    Original-patch-by: default avatarOleg Nesterov <oleg@redhat.com>
    Original-patch-by: default avatarMichal Hocko <mhocko@suse.com>
    Signed-off-by: default avatarHugh Dickins <hughd@google.com>
    Acked-by: default avatarMichal Hocko <mhocko@suse.com>
    Tested-by: Helge Deller <deller@gmx.de> # parisc
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    [wt: backport to 4.11: adjust context]
    [wt: backport to 4.9: adjust context ; kernel doc was not in admin-guide]
    Signed-off-by: default avatarWilly Tarreau <w@1wt.eu>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    cfc0eb40
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