From: Samuel Holland <[email protected]> Currently, kasan_mem_to_shadow() uses a logical right shift, which turns canonical kernel addresses into non-canonical addresses by clearing the high KASAN_SHADOW_SCALE_SHIFT bits. The value of KASAN_SHADOW_OFFSET is then chosen so that the addition results in a canonical address for the shadow memory.
For KASAN_GENERIC, this shift/add combination is ABI with the compiler, because KASAN_SHADOW_OFFSET is used in compiler-generated inline tag checks[1], which must only attempt to dereference canonical addresses. However, for KASAN_SW_TAGS there is some freedom to change the algorithm without breaking the ABI. Because TBI is enabled for kernel addresses, the top bits of shadow memory addresses computed during tag checks are irrelevant, and so likewise are the top bits of KASAN_SHADOW_OFFSET. This is demonstrated by the fact that LLVM uses a logical right shift in the tag check fast path[2] but a sbfx (signed bitfield extract) instruction in the slow path[3] without causing any issues. Use an arithmetic shift in kasan_mem_to_shadow() as it provides a number of benefits: 1) The memory layout doesn't change but is easier to understand. KASAN_SHADOW_OFFSET becomes a canonical memory address, and the shifted pointer becomes a negative offset, so KASAN_SHADOW_OFFSET == KASAN_SHADOW_END regardless of the shift amount or the size of the virtual address space. 2) KASAN_SHADOW_OFFSET becomes a simpler constant, requiring only one instruction to load instead of two. Since it must be loaded in each function with a tag check, this decreases kernel text size by 0.5%. 3) This shift and the sign extension from kasan_reset_tag() can be combined into a single sbfx instruction. When this same algorithm change is applied to the compiler, it removes an instruction from each inline tag check, further reducing kernel text size by an additional 4.6%. These benefits extend to other architectures as well. On RISC-V, where the baseline ISA does not shifted addition or have an equivalent to the sbfx instruction, loading KASAN_SHADOW_OFFSET is reduced from 3 to 2 instructions, and kasan_mem_to_shadow(kasan_reset_tag(addr)) similarly combines two consecutive right shifts. Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/AddressSanitizer.cpp#L1316 [1] Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Transforms/Instrumentation/HWAddressSanitizer.cpp#L895 [2] Link: https://github.com/llvm/llvm-project/blob/llvmorg-20-init/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp#L669 [3] Signed-off-by: Samuel Holland <[email protected]> Co-developed-by: Maciej Wieczor-Retman <[email protected]> Signed-off-by: Maciej Wieczor-Retman <[email protected]> --- Changelog v11: (Maciej) - Remove the arch_kasan_non_canonical_hook() scheme in favor of Andrey Ryabinin's much nicer simple implementation. Changelog v10: (Maciej) - Update the Documentation/dev-tools/kasan.rst file with the changed kasan_mem_to_shadow(). Changelog v9: (Maciej) - Take out the arm64 related code from mm/kasan/report.c and put it in the arch specific directory in a new file so the kasan_mem_to_shadow() function can be included. - Reset addr tag bits in arm64's arch_kasan_non_canonical_hook() so the inline mode can also work with that function (Andrey Ryabinin). - Fix incorrect number of zeros in a comment in mm/kasan/report.c. - Remove Catalin's acked-by since changes were made. Changelog v7: (Maciej) - Change UL to ULL in report.c to fix some compilation warnings. Changelog v6: (Maciej) - Add Catalin's acked-by. - Move x86 gdb snippet here from the last patch. Changelog v5: (Maciej) - (u64) -> (unsigned long) in report.c Changelog v4: (Maciej) - Revert x86 to signed mem_to_shadow mapping. - Remove last two paragraphs since they were just poorer duplication of the comments in kasan_non_canonical_hook(). Changelog v3: (Maciej) - Fix scripts/gdb/linux/kasan.py so the new signed mem_to_shadow() is reflected there. - Fix Documentation/arch/arm64/kasan-offsets.sh to take new offsets into account. - Made changes to the kasan_non_canonical_hook() according to upstream discussion. Settled on overflow on both ranges and separate checks for x86 and arm. Changelog v2: (Maciej) - Correct address range that's checked in kasan_non_canonical_hook(). Adjust the comment inside. - Remove part of comment from arch/arm64/include/asm/memory.h. - Append patch message paragraph about the overflow in kasan_non_canonical_hook(). Documentation/arch/arm64/kasan-offsets.sh | 8 ++++++-- Documentation/dev-tools/kasan.rst | 18 ++++++++++++------ arch/arm64/Kconfig | 10 +++++----- arch/arm64/include/asm/memory.h | 14 +++++++++++++- arch/arm64/mm/kasan_init.c | 7 +++++-- include/linux/kasan.h | 10 ++++++++-- mm/kasan/report.c | 16 ++++++++++++---- scripts/gdb/linux/kasan.py | 5 ++++- scripts/gdb/linux/mm.py | 5 +++-- 9 files changed, 68 insertions(+), 25 deletions(-) diff --git a/Documentation/arch/arm64/kasan-offsets.sh b/Documentation/arch/arm64/kasan-offsets.sh index 2dc5f9e18039..ce777c7c7804 100644 --- a/Documentation/arch/arm64/kasan-offsets.sh +++ b/Documentation/arch/arm64/kasan-offsets.sh @@ -5,8 +5,12 @@ print_kasan_offset () { printf "%02d\t" $1 - printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \ - - (1 << (64 - 32 - $2)) )) + if [[ $2 -ne 4 ]] then + printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) \ + - (1 << (64 - 32 - $2)) )) + else + printf "0x%08x00000000\n" $(( (0xffffffff & (-1 << ($1 - 1 - 32))) )) + fi } echo KASAN_SHADOW_SCALE_SHIFT = 3 diff --git a/Documentation/dev-tools/kasan.rst b/Documentation/dev-tools/kasan.rst index 4968b2aa60c8..b11c1be8dff4 100644 --- a/Documentation/dev-tools/kasan.rst +++ b/Documentation/dev-tools/kasan.rst @@ -315,13 +315,19 @@ translate a memory address to its corresponding shadow address. Here is the function which translates an address to its corresponding shadow address:: - static inline void *kasan_mem_to_shadow(const void *addr) - { - return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT) - + KASAN_SHADOW_OFFSET; - } + static inline void *kasan_mem_to_shadow(const void *addr) + { + void *scaled; -where ``KASAN_SHADOW_SCALE_SHIFT = 3``. + if (IS_ENABLED(CONFIG_KASAN_GENERIC)) + scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT); + else + scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT); + + return KASAN_SHADOW_OFFSET + scaled; + } + +where for Generic KASAN ``KASAN_SHADOW_SCALE_SHIFT = 3``. Compile-time instrumentation is used to insert memory access checks. Compiler inserts function calls (``__asan_load*(addr)``, ``__asan_store*(addr)``) before diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index 38dba5f7e4d2..4de7e721f32e 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -435,11 +435,11 @@ config KASAN_SHADOW_OFFSET default 0xdffffe0000000000 if ARM64_VA_BITS_42 && !KASAN_SW_TAGS default 0xdfffffc000000000 if ARM64_VA_BITS_39 && !KASAN_SW_TAGS default 0xdffffff800000000 if ARM64_VA_BITS_36 && !KASAN_SW_TAGS - default 0xefff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && KASAN_SW_TAGS - default 0xefffc00000000000 if (ARM64_VA_BITS_47 || ARM64_VA_BITS_52) && ARM64_16K_PAGES && KASAN_SW_TAGS - default 0xeffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS - default 0xefffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS - default 0xeffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS + default 0xffff800000000000 if (ARM64_VA_BITS_48 || (ARM64_VA_BITS_52 && !ARM64_16K_PAGES)) && KASAN_SW_TAGS + default 0xffffc00000000000 if (ARM64_VA_BITS_47 || ARM64_VA_BITS_52) && ARM64_16K_PAGES && KASAN_SW_TAGS + default 0xfffffe0000000000 if ARM64_VA_BITS_42 && KASAN_SW_TAGS + default 0xffffffc000000000 if ARM64_VA_BITS_39 && KASAN_SW_TAGS + default 0xfffffff800000000 if ARM64_VA_BITS_36 && KASAN_SW_TAGS default 0xffffffffffffffff config UNWIND_TABLES diff --git a/arch/arm64/include/asm/memory.h b/arch/arm64/include/asm/memory.h index a2b7a33966ff..875c0bd0d85a 100644 --- a/arch/arm64/include/asm/memory.h +++ b/arch/arm64/include/asm/memory.h @@ -89,7 +89,15 @@ * * KASAN_SHADOW_END is defined first as the shadow address that corresponds to * the upper bound of possible virtual kernel memory addresses UL(1) << 64 - * according to the mapping formula. + * according to the mapping formula. For Generic KASAN, the address in the + * mapping formula is treated as unsigned (part of the compiler's ABI), so the + * end of the shadow memory region is at a large positive offset from + * KASAN_SHADOW_OFFSET. For Software Tag-Based KASAN, the address in the + * formula is treated as signed. Since all kernel addresses are negative, they + * map to shadow memory below KASAN_SHADOW_OFFSET, making KASAN_SHADOW_OFFSET + * itself the end of the shadow memory region. (User pointers are positive and + * would map to shadow memory above KASAN_SHADOW_OFFSET, but shadow memory is + * not allocated for them.) * * KASAN_SHADOW_START is defined second based on KASAN_SHADOW_END. The shadow * memory start must map to the lowest possible kernel virtual memory address @@ -100,7 +108,11 @@ */ #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS) #define KASAN_SHADOW_OFFSET _AC(CONFIG_KASAN_SHADOW_OFFSET, UL) +#ifdef CONFIG_KASAN_GENERIC #define KASAN_SHADOW_END ((UL(1) << (64 - KASAN_SHADOW_SCALE_SHIFT)) + KASAN_SHADOW_OFFSET) +#else +#define KASAN_SHADOW_END KASAN_SHADOW_OFFSET +#endif #define _KASAN_SHADOW_START(va) (KASAN_SHADOW_END - (UL(1) << ((va) - KASAN_SHADOW_SCALE_SHIFT))) #define KASAN_SHADOW_START _KASAN_SHADOW_START(vabits_actual) #define PAGE_END KASAN_SHADOW_START diff --git a/arch/arm64/mm/kasan_init.c b/arch/arm64/mm/kasan_init.c index abeb81bf6ebd..937f6eb8115b 100644 --- a/arch/arm64/mm/kasan_init.c +++ b/arch/arm64/mm/kasan_init.c @@ -198,8 +198,11 @@ static bool __init root_level_aligned(u64 addr) /* The early shadow maps everything to a single page of zeroes */ asmlinkage void __init kasan_early_init(void) { - BUILD_BUG_ON(KASAN_SHADOW_OFFSET != - KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT))); + if (IS_ENABLED(CONFIG_KASAN_GENERIC)) + BUILD_BUG_ON(KASAN_SHADOW_OFFSET != + KASAN_SHADOW_END - (1UL << (64 - KASAN_SHADOW_SCALE_SHIFT))); + else + BUILD_BUG_ON(KASAN_SHADOW_OFFSET != KASAN_SHADOW_END); BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS), SHADOW_ALIGN)); BUILD_BUG_ON(!IS_ALIGNED(_KASAN_SHADOW_START(VA_BITS_MIN), SHADOW_ALIGN)); BUILD_BUG_ON(!IS_ALIGNED(KASAN_SHADOW_END, SHADOW_ALIGN)); diff --git a/include/linux/kasan.h b/include/linux/kasan.h index 338a1921a50a..81c83dcfcebe 100644 --- a/include/linux/kasan.h +++ b/include/linux/kasan.h @@ -62,8 +62,14 @@ int kasan_populate_early_shadow(const void *shadow_start, #ifndef kasan_mem_to_shadow static inline void *kasan_mem_to_shadow(const void *addr) { - return (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT) - + KASAN_SHADOW_OFFSET; + void *scaled; + + if (IS_ENABLED(CONFIG_KASAN_GENERIC)) + scaled = (void *)((unsigned long)addr >> KASAN_SHADOW_SCALE_SHIFT); + else + scaled = (void *)((long)addr >> KASAN_SHADOW_SCALE_SHIFT); + + return KASAN_SHADOW_OFFSET + scaled; } #endif diff --git a/mm/kasan/report.c b/mm/kasan/report.c index e804b1e1f886..1e4521b5ef14 100644 --- a/mm/kasan/report.c +++ b/mm/kasan/report.c @@ -640,12 +640,20 @@ void kasan_non_canonical_hook(unsigned long addr) { unsigned long orig_addr, user_orig_addr; const char *bug_type; + void *tagged_null = set_tag(NULL, KASAN_TAG_KERNEL); + void *tagged_addr = set_tag((void *)addr, KASAN_TAG_KERNEL); /* - * All addresses that came as a result of the memory-to-shadow mapping - * (even for bogus pointers) must be >= KASAN_SHADOW_OFFSET. + * Filter out addresses that cannot be shadow memory accesses generated + * by the compiler. + * + * In SW_TAGS mode, when computing a shadow address, the compiler always + * sets the kernel tag (some top bits) on the pointer *before* computing + * the memory-to-shadow mapping. As a result, valid shadow addresses + * are derived from tagged kernel pointers. */ - if (addr < KASAN_SHADOW_OFFSET) + if (tagged_addr < kasan_mem_to_shadow(tagged_null) || + tagged_addr > kasan_mem_to_shadow((void *)(~0ULL))) return; orig_addr = (unsigned long)kasan_shadow_to_mem((void *)addr); @@ -670,7 +678,7 @@ void kasan_non_canonical_hook(unsigned long addr) } else if (user_orig_addr < TASK_SIZE) { bug_type = "probably user-memory-access"; orig_addr = user_orig_addr; - } else if (addr_in_shadow((void *)addr)) + } else if (addr_in_shadow(tagged_addr)) bug_type = "probably wild-memory-access"; else bug_type = "maybe wild-memory-access"; diff --git a/scripts/gdb/linux/kasan.py b/scripts/gdb/linux/kasan.py index 56730b3fde0b..4b86202b155f 100644 --- a/scripts/gdb/linux/kasan.py +++ b/scripts/gdb/linux/kasan.py @@ -7,7 +7,8 @@ # import gdb -from linux import constants, mm +from linux import constants, utils, mm +from ctypes import c_int64 as s64 def help(): t = """Usage: lx-kasan_mem_to_shadow [Hex memory addr] @@ -39,6 +40,8 @@ class KasanMemToShadow(gdb.Command): else: help() def kasan_mem_to_shadow(self, addr): + if constants.CONFIG_KASAN_SW_TAGS and not utils.is_target_arch('x86'): + addr = s64(addr) return (addr >> self.p_ops.KASAN_SHADOW_SCALE_SHIFT) + self.p_ops.KASAN_SHADOW_OFFSET KasanMemToShadow() diff --git a/scripts/gdb/linux/mm.py b/scripts/gdb/linux/mm.py index d78908f6664d..d4ab341d89c5 100644 --- a/scripts/gdb/linux/mm.py +++ b/scripts/gdb/linux/mm.py @@ -281,12 +281,13 @@ class aarch64_page_ops(): self.KERNEL_END = gdb.parse_and_eval("_end") if constants.LX_CONFIG_KASAN_GENERIC or constants.LX_CONFIG_KASAN_SW_TAGS: + self.KASAN_SHADOW_OFFSET = constants.LX_CONFIG_KASAN_SHADOW_OFFSET if constants.LX_CONFIG_KASAN_GENERIC: self.KASAN_SHADOW_SCALE_SHIFT = 3 + self.KASAN_SHADOW_END = (1 << (64 - self.KASAN_SHADOW_SCALE_SHIFT)) + self.KASAN_SHADOW_OFFSET else: self.KASAN_SHADOW_SCALE_SHIFT = 4 - self.KASAN_SHADOW_OFFSET = constants.LX_CONFIG_KASAN_SHADOW_OFFSET - self.KASAN_SHADOW_END = (1 << (64 - self.KASAN_SHADOW_SCALE_SHIFT)) + self.KASAN_SHADOW_OFFSET + self.KASAN_SHADOW_END = self.KASAN_SHADOW_OFFSET self.PAGE_END = self.KASAN_SHADOW_END - (1 << (self.vabits_actual - self.KASAN_SHADOW_SCALE_SHIFT)) else: self.PAGE_END = self._PAGE_END(self.VA_BITS_MIN) -- 2.53.0
