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872bb37f68
The codgen for adding architecture-specific stack alignment to the effective alloca() usage is somewhat inefficient and allows a bit to get carried beyond the desired entropy range. This isn't really a problem, but it's unexpected and the codegen is kind of bad. Quoting Mark[1], the disassembly for arm64's invoke_syscall() looks like: // offset = raw_cpu_read(kstack_offset) mov x4, sp adrp x0, kstack_offset mrs x5, tpidr_el1 add x0, x0, #:lo12:kstack_offset ldr w0, [x0, x5] // offset = KSTACK_OFFSET_MAX(offset) and x0, x0, #0x3ff // alloca(offset) add x0, x0, #0xf and x0, x0, #0x7f0 sub sp, x4, x0 ... which in C would be: offset = raw_cpu_read(kstack_offset) offset &= 0x3ff; // [0x0, 0x3ff] offset += 0xf; // [0xf, 0x40e] offset &= 0x7f0; // [0x0, ... so when *all* bits [3:0] are 0, they'll have no impact, and when *any* of bits [3:0] are 1 they'll trigger a carry into bit 4, which could ripple all the way up and spill into bit 10. Switch the masking in KSTACK_OFFSET_MAX() to explicitly clear the bottom bits to avoid the rounding by using 0b1111110000 instead of 0b1111111111: // offset = raw_cpu_read(kstack_offset) mov x4, sp adrp x0, 0 <kstack_offset> mrs x5, tpidr_el1 add x0, x0, #:lo12:kstack_offset ldr w0, [x0, x5] // offset = KSTACK_OFFSET_MAX(offset) and x0, x0, #0x3f0 // alloca(offset) sub sp, x4, x0 Suggested-by: Mark Rutland <mark.rutland@arm.com> Link: https://lore.kernel.org/lkml/ZnVfOnIuFl2kNWkT@J2N7QTR9R3/ [1] Link: https://lore.kernel.org/r/20240702211612.work.576-kees@kernel.org Signed-off-by: Kees Cook <kees@kernel.org> |
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| acpi | ||
| asm-generic | ||
| clocksource | ||
| crypto | ||
| drm | ||
| dt-bindings | ||
| keys | ||
| kunit | ||
| kvm | ||
| linux | ||
| math-emu | ||
| media | ||
| memory | ||
| misc | ||
| net | ||
| pcmcia | ||
| ras | ||
| rdma | ||
| rv | ||
| scsi | ||
| soc | ||
| sound | ||
| target | ||
| trace | ||
| uapi | ||
| ufs | ||
| vdso | ||
| video | ||
| xen | ||