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SubjectRe: [PATCH v2] arm64: bpf: Fix branch offset in JIT
On Tue, Sep 15, 2020 at 04:53:44PM +0300, Ilias Apalodimas wrote:
> On Tue, Sep 15, 2020 at 02:11:03PM +0100, Will Deacon wrote:
> > Hi Ilias,
> >
> > On Mon, Sep 14, 2020 at 07:03:55PM +0300, Ilias Apalodimas wrote:
> > > Running the eBPF test_verifier leads to random errors looking like this:
> > >
> > > [ 6525.735488] Unexpected kernel BRK exception at EL1
> > > [ 6525.735502] Internal error: ptrace BRK handler: f2000100 [#1] SMP
> >
> > Does this happen because we poison the BPF memory with BRK instructions?
> > Maybe we should look at using a special immediate so we can detect this,
> > rather than end up in the ptrace handler.
>
> As discussed offline this is what aarch64_insn_gen_branch_imm() will return for
> offsets > 128M and yes replacing the handler with a more suitable message would
> be good.

Can you give the diff below a shot, please? Hopefully printing a more useful
message will mean these things get triaged/debugged better in future.

Will

--->8

diff --git a/arch/arm64/include/asm/extable.h b/arch/arm64/include/asm/extable.h
index 840a35ed92ec..b15eb4a3e6b2 100644
--- a/arch/arm64/include/asm/extable.h
+++ b/arch/arm64/include/asm/extable.h
@@ -22,6 +22,15 @@ struct exception_table_entry

#define ARCH_HAS_RELATIVE_EXTABLE

+static inline bool in_bpf_jit(struct pt_regs *regs)
+{
+ if (!IS_ENABLED(CONFIG_BPF_JIT))
+ return false;
+
+ return regs->pc >= BPF_JIT_REGION_START &&
+ regs->pc < BPF_JIT_REGION_END;
+}
+
#ifdef CONFIG_BPF_JIT
int arm64_bpf_fixup_exception(const struct exception_table_entry *ex,
struct pt_regs *regs);
diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index 7310a4f7f993..fa76151de6ff 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -384,7 +384,7 @@ void __init debug_traps_init(void)
hook_debug_fault_code(DBG_ESR_EVT_HWSS, single_step_handler, SIGTRAP,
TRAP_TRACE, "single-step handler");
hook_debug_fault_code(DBG_ESR_EVT_BRK, brk_handler, SIGTRAP,
- TRAP_BRKPT, "ptrace BRK handler");
+ TRAP_BRKPT, "BRK handler");
}

/* Re-enable single step for syscall restarting. */
diff --git a/arch/arm64/kernel/traps.c b/arch/arm64/kernel/traps.c
index 13ebd5ca2070..9f7fde99eda2 100644
--- a/arch/arm64/kernel/traps.c
+++ b/arch/arm64/kernel/traps.c
@@ -34,6 +34,7 @@
#include <asm/daifflags.h>
#include <asm/debug-monitors.h>
#include <asm/esr.h>
+#include <asm/extable.h>
#include <asm/insn.h>
#include <asm/kprobes.h>
#include <asm/traps.h>
@@ -994,6 +995,21 @@ static struct break_hook bug_break_hook = {
.imm = BUG_BRK_IMM,
};

+static int reserved_fault_handler(struct pt_regs *regs, unsigned int esr)
+{
+ pr_err("%s generated an invalid instruction at %pS!\n",
+ in_bpf_jit(regs) ? "BPF JIT" : "Kernel runtime patching",
+ instruction_pointer(regs));
+
+ /* We cannot handle this */
+ return DBG_HOOK_ERROR;
+}
+
+static struct break_hook fault_break_hook = {
+ .fn = reserved_fault_handler,
+ .imm = FAULT_BRK_IMM,
+};
+
#ifdef CONFIG_KASAN_SW_TAGS

#define KASAN_ESR_RECOVER 0x20
@@ -1059,6 +1075,7 @@ int __init early_brk64(unsigned long addr, unsigned int esr,
void __init trap_init(void)
{
register_kernel_break_hook(&bug_break_hook);
+ register_kernel_break_hook(&fault_break_hook);
#ifdef CONFIG_KASAN_SW_TAGS
register_kernel_break_hook(&kasan_break_hook);
#endif
diff --git a/arch/arm64/mm/extable.c b/arch/arm64/mm/extable.c
index eee1732ab6cd..aa0060178343 100644
--- a/arch/arm64/mm/extable.c
+++ b/arch/arm64/mm/extable.c
@@ -14,9 +14,7 @@ int fixup_exception(struct pt_regs *regs)
if (!fixup)
return 0;

- if (IS_ENABLED(CONFIG_BPF_JIT) &&
- regs->pc >= BPF_JIT_REGION_START &&
- regs->pc < BPF_JIT_REGION_END)
+ if (in_bpf_jit(regs))
return arm64_bpf_fixup_exception(fixup, regs);

regs->pc = (unsigned long)&fixup->fixup + fixup->fixup;
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