From df7e1286b1dc3d6cff952cf3ef4f0c36831e2fbb Mon Sep 17 00:00:00 2001 From: Mark Brown Date: Wed, 4 Sep 2024 17:58:01 +0100 Subject: [PATCH] mm: care about shadow stack guard gap when getting an unmapped area As covered in the commit log for c44357c2e76b ("x86/mm: care about shadow stack guard gap during placement") our current mmap() implementation does not take care to ensure that a new mapping isn't placed with existing mappings inside it's own guard gaps. This is particularly important for shadow stacks since if two shadow stacks end up getting placed adjacent to each other then they can overflow into each other which weakens the protection offered by the feature. On x86 there is a custom arch_get_unmapped_area() which was updated by the above commit to cover this case by specifying a start_gap for allocations with VM_SHADOW_STACK. Both arm64 and RISC-V have equivalent features and use the generic implementation of arch_get_unmapped_area() so let's make the equivalent change there so they also don't get shadow stack pages placed without guard pages. x86 uses a single page guard, this is also sufficient for arm64 where we either do single word pops and pushes or unconstrained writes. Architectures which do not have this feature will define VM_SHADOW_STACK to VM_NONE and hence be unaffected. Link: https://lkml.kernel.org/r/20240904-mm-generic-shadow-stack-guard-v2-3-a46b8b6dc0ed@kernel.org Signed-off-by: Mark Brown Suggested-by: Rick Edgecombe Acked-by: Lorenzo Stoakes Cc: Alexander Gordeev Cc: Andreas Larsson Cc: Borislav Petkov Cc: Christian Borntraeger Cc: Christophe Leroy Cc: Chris Zankel Cc: Dave Hansen Cc: David S. Miller Cc: Gerald Schaefer Cc: Guo Ren Cc: Heiko Carstens Cc: Helge Deller Cc: "H. Peter Anvin" Cc: Huacai Chen Cc: Ingo Molnar Cc: Ivan Kokshaysky Cc: James Bottomley Cc: John Paul Adrian Glaubitz Cc: Liam R. Howlett Cc: Matt Turner Cc: Max Filippov Cc: Michael Ellerman Cc: Naveen N Rao Cc: Nicholas Piggin Cc: Richard Henderson Cc: Rich Felker Cc: Russell King Cc: Sven Schnelle Cc: Thomas Bogendoerfer Cc: Thomas Gleixner Cc: Vasily Gorbik Cc: Vineet Gupta Cc: Vlastimil Babka Cc: WANG Xuerui Cc: Yoshinori Sato Signed-off-by: Andrew Morton --- mm/mmap.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) diff --git a/mm/mmap.c b/mm/mmap.c index 14d36be65ce7..270bd8d504b8 100644 --- a/mm/mmap.c +++ b/mm/mmap.c @@ -702,6 +702,18 @@ retry: return gap; } +/* + * Determine if the allocation needs to ensure that there is no + * existing mapping within it's guard gaps, for use as start_gap. + */ +static inline unsigned long stack_guard_placement(vm_flags_t vm_flags) +{ + if (vm_flags & VM_SHADOW_STACK) + return PAGE_SIZE; + + return 0; +} + /* * Search for an unmapped address range. * @@ -763,6 +775,7 @@ generic_get_unmapped_area(struct file *filp, unsigned long addr, info.length = len; info.low_limit = mm->mmap_base; info.high_limit = mmap_end; + info.start_gap = stack_guard_placement(vm_flags); return vm_unmapped_area(&info); } @@ -812,6 +825,7 @@ generic_get_unmapped_area_topdown(struct file *filp, unsigned long addr, info.length = len; info.low_limit = PAGE_SIZE; info.high_limit = arch_get_mmap_base(addr, mm->mmap_base); + info.start_gap = stack_guard_placement(vm_flags); addr = vm_unmapped_area(&info); /* -- 2.49.0