[PATCH 5.15 367/691] mm: only enforce minimum stack gap size if its sensible

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5.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: David Gow <davidgow@xxxxxxxxxx>

commit 69b50d4351ed924f29e3d46b159e28f70dfc707f upstream.

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@xxxxxxxxxx
Fixes: dba79c3df4a2 ("arm: use generic mmap top-down layout and brk randomization")
Signed-off-by: David Gow <davidgow@xxxxxxxxxx>
Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
Cc: Alexandre Ghiti <alex@xxxxxxxx>
Cc: Linus Walleij <linus.walleij@xxxxxxxxxx>
Cc: Luis Chamberlain <mcgrof@xxxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Russell King <linux@xxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/util.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/util.c
+++ b/mm/util.c
@@ -430,7 +430,7 @@ static unsigned long mmap_base(unsigned
 	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;






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