mm: only enforce minimum stack gap size if it's sensible

The generic mmap_base code tries to leave a gap between the top of the
stack and the mmap base address, but enforces a minimum gap size (MIN_GAP)
of 128MB, which is too large on some setups.  In particular, on arm tasks
without ADDR_LIMIT_32BIT, the STACK_TOP value is less than 128MB, so it's
impossible to fit such a gap in.

Only enforce this minimum if MIN_GAP < MAX_GAP, as we'd prefer to honour
MAX_GAP, which is defined proportionally, so scales better and always
leaves us with both _some_ stack space and some room for mmap.

This fixes the usercopy KUnit test suite on 32-bit arm, as it doesn't set
any personality flags so gets the default (in this case 26-bit) task size.
This test can be run with: ./tools/testing/kunit/kunit.py run --arch arm
usercopy --make_options LLVM=1

Link: https://lkml.kernel.org/r/20240803074642.1849623-2-davidgow@google.com
Fixes: dba79c3df4a2 ("arm: use generic mmap top-down layout and brk randomization")
Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/util.c b/mm/util.c
index 678c647b778ff9669f24b6af418043847316ec5b..2bfae12ade7e6893442467af0c0c833ba71678db 100644 (file)
--- a/mm/util.c
+++ b/mm/util.c
@@ -463,7 +463,7 @@ static unsigned long mmap_base(unsigned long rnd, struct rlimit *rlim_stack)
        if (gap + pad > gap)
                gap += pad;
 
-       if (gap < MIN_GAP)
+       if (gap < MIN_GAP && MIN_GAP < MAX_GAP)
                gap = MIN_GAP;
        else if (gap > MAX_GAP)
                gap = MAX_GAP;