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authorPeter Maydell <peter.maydell@linaro.org>2018-05-22 17:11:20 +0100
committerWill Deacon <will.deacon@arm.com>2018-05-22 17:14:20 +0100
commitcc19846079a70abcfd91b5a0791a5f17d69458a5 (patch)
tree691577dde2aa215a0794f710b10d626f5bd8d656 /arch
parent255845fc43a3aaf806852a1d3bc89bff1411ebe3 (diff)
arm64: fault: Don't leak data in ESR context for user fault on kernel VA
If userspace faults on a kernel address, handing them the raw ESR value on the sigframe as part of the delivered signal can leak data useful to attackers who are using information about the underlying hardware fault type (e.g. translation vs permission) as a mechanism to defeat KASLR. However there are also legitimate uses for the information provided in the ESR -- notably the GCC and LLVM sanitizers use this to report whether wild pointer accesses by the application are reads or writes (since a wild write is a more serious bug than a wild read), so we don't want to drop the ESR information entirely. For faulting addresses in the kernel, sanitize the ESR. We choose to present userspace with the illusion that there is nothing mapped in the kernel's part of the address space at all, by reporting all faults as level 0 translation faults taken to EL1. These fields are safe to pass through to userspace as they depend only on the instruction that userspace used to provoke the fault: EC IL (always) ISV CM WNR (for all data aborts) All the other fields in ESR except DFSC are architecturally RES0 for an L0 translation fault taken to EL1, so can be zeroed out without confusing userspace. The illusion is not entirely perfect, as there is a tiny wrinkle where we will report an alignment fault that was not due to the memory type (for instance a LDREX to an unaligned address) as a translation fault, whereas if you do this on real unmapped memory the alignment fault takes precedence. This is not likely to trip anybody up in practice, as the only users we know of for the ESR information who care about the behaviour for kernel addresses only really want to know about the WnR bit. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Signed-off-by: Will Deacon <will.deacon@arm.com>
Diffstat (limited to 'arch')
-rw-r--r--arch/arm64/mm/fault.c51
1 files changed, 51 insertions, 0 deletions
diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 4165485e8b6e..2af3dd89bcdb 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -293,6 +293,57 @@ static void __do_kernel_fault(unsigned long addr, unsigned int esr,
static void __do_user_fault(struct siginfo *info, unsigned int esr)
{
current->thread.fault_address = (unsigned long)info->si_addr;
+
+ /*
+ * If the faulting address is in the kernel, we must sanitize the ESR.
+ * From userspace's point of view, kernel-only mappings don't exist
+ * at all, so we report them as level 0 translation faults.
+ * (This is not quite the way that "no mapping there at all" behaves:
+ * an alignment fault not caused by the memory type would take
+ * precedence over translation fault for a real access to empty
+ * space. Unfortunately we can't easily distinguish "alignment fault
+ * not caused by memory type" from "alignment fault caused by memory
+ * type", so we ignore this wrinkle and just return the translation
+ * fault.)
+ */
+ if (current->thread.fault_address >= TASK_SIZE) {
+ switch (ESR_ELx_EC(esr)) {
+ case ESR_ELx_EC_DABT_LOW:
+ /*
+ * These bits provide only information about the
+ * faulting instruction, which userspace knows already.
+ * We explicitly clear bits which are architecturally
+ * RES0 in case they are given meanings in future.
+ * We always report the ESR as if the fault was taken
+ * to EL1 and so ISV and the bits in ISS[23:14] are
+ * clear. (In fact it always will be a fault to EL1.)
+ */
+ esr &= ESR_ELx_EC_MASK | ESR_ELx_IL |
+ ESR_ELx_CM | ESR_ELx_WNR;
+ esr |= ESR_ELx_FSC_FAULT;
+ break;
+ case ESR_ELx_EC_IABT_LOW:
+ /*
+ * Claim a level 0 translation fault.
+ * All other bits are architecturally RES0 for faults
+ * reported with that DFSC value, so we clear them.
+ */
+ esr &= ESR_ELx_EC_MASK | ESR_ELx_IL;
+ esr |= ESR_ELx_FSC_FAULT;
+ break;
+ default:
+ /*
+ * This should never happen (entry.S only brings us
+ * into this code for insn and data aborts from a lower
+ * exception level). Fail safe by not providing an ESR
+ * context record at all.
+ */
+ WARN(1, "ESR 0x%x is not DABT or IABT from EL0\n", esr);
+ esr = 0;
+ break;
+ }
+ }
+
current->thread.fault_code = esr;
arm64_force_sig_info(info, esr_to_fault_info(esr)->name, current);
}