[PATCH] kprobes/arm: fix emulation of LDR/STR instruction when Rn == PC
From: nico@fluxnic.net (Nicolas Pitre)
Date: 2011-03-26 02:03:07
On Fri, 25 Mar 2011, Viktor Rosendahl wrote:
The Rn value from the emulation is unconditionally written back; this is fine as long as Rn != PC because in that case, even if the instruction isn't a write back instruction, it will only result in the same value being written back. In case Rn == PC, then the emulated instruction doesn't have the actual PC value in Rn but an adjusted value; when this is written back, it will result in the PC being incorrectly updated. An altenative solution would be to check bits 24 and 22 to see whether the instruction actually is a write back instruction or not. I think it's enough to check whether Rn != PC, because: - it's looks cheaper than the alternative - to my understaning it's not permitted to update the PC with a write back instruction, so we don't lose any ability to emulate legal instructions. - in case of writing back for non write back instructions where Rn != PC, it doesn't matter because the values are the same. Regarding the second point above, it would possibly be prudent to add some checking to prep_emulate_ldr_str(), so that instructions with write back and Rn == PC would be rejected. Signed-off-by: Viktor Rosendahl <redacted>
Acked-by: Nicolas Pitre <redacted> Please send to RMK's patch system.
quoted hunk ↗ jump to hunk
--- arch/arm/kernel/kprobes-decode.c | 10 ++++++---- 1 files changed, 6 insertions(+), 4 deletions(-)diff --git a/arch/arm/kernel/kprobes-decode.c b/arch/arm/kernel/kprobes-decode.c index 8f6ed43..2389131 100644 --- a/arch/arm/kernel/kprobes-decode.c +++ b/arch/arm/kernel/kprobes-decode.c@@ -594,7 +594,8 @@ static void __kprobes emulate_ldr(struct kprobe *p, struct pt_regs *regs) long cpsr = regs->ARM_cpsr; fnr.dr = insnslot_llret_3arg_rflags(rnv, 0, rmv, cpsr, i_fn); - regs->uregs[rn] = fnr.r0; /* Save Rn in case of writeback. */ + if (rn != 15) + regs->uregs[rn] = fnr.r0; /* Save Rn in case of writeback. */ rdv = fnr.r1; if (rd == 15) {@@ -622,10 +623,11 @@ static void __kprobes emulate_str(struct kprobe *p, struct pt_regs *regs) long rdv = (rd == 15) ? iaddr + str_pc_offset : regs->uregs[rd]; long rnv = (rn == 15) ? iaddr + 8 : regs->uregs[rn]; long rmv = regs->uregs[rm]; /* rm/rmv may be invalid, don't care. */ + long rnv_wb; - /* Save Rn in case of writeback. */ - regs->uregs[rn] = - insnslot_3arg_rflags(rnv, rdv, rmv, regs->ARM_cpsr, i_fn); + rnv_wb = insnslot_3arg_rflags(rnv, rdv, rmv, regs->ARM_cpsr, i_fn); + if (rn != 15) + regs->uregs[rn] = rnv_wb; /* Save Rn in case of writeback. */ } static void __kprobes emulate_mrrc(struct kprobe *p, struct pt_regs *regs)-- 1.7.2.5 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel at lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel