x86/kprobes: Verify stack frame on kretprobe [Linux 5.1]

x86/kprobes: Verify stack frame on kretprobe [Linux 5.1]

This Linux kernel change "x86/kprobes: Verify stack frame on kretprobe" is included in the Linux 5.1 release. This change is authored by Masami Hiramatsu <mhiramat [at] kernel.org> on Sun Feb 24 01:49:52 2019 +0900. The commit for this change in Linux stable tree is 3ff9c07 (patch).

x86/kprobes: Verify stack frame on kretprobe

Verify the stack frame pointer on kretprobe trampoline handler,
If the stack frame pointer does not match, it skips the wrong
entry and tries to find correct one.

This can happen if user puts the kretprobe on the function
which can be used in the path of ftrace user-function call.
Such functions should not be probed, so this adds a warning
message that reports which function should be blacklisted.

Tested-by: Andrea Righi <righi.andrea@gmail.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/155094059185.6137.15527904013362842072.stgit@devbox
Signed-off-by: Ingo Molnar <mingo@kernel.org>

There are 27 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 arch/x86/kernel/kprobes/core.c | 26 ++++++++++++++++++++++++++
 include/linux/kprobes.h        |  1 +
 2 files changed, 27 insertions(+)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index a034cb8..18fbe9b 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -569,6 +569,7 @@ void arch_prepare_kretprobe(struct kretprobe_instance *ri, struct pt_regs *regs)
    unsigned long *sara = stack_addr(regs);

    ri->ret_addr = (kprobe_opcode_t *) *sara;
+   ri->fp = sara;

    /* Replace the return addr with trampoline addr */
    *sara = (unsigned long) &kretprobe_trampoline;
@@ -759,15 +760,21 @@ static __used void *trampoline_handler(struct pt_regs *regs)
    unsigned long flags, orig_ret_address = 0;
    unsigned long trampoline_address = (unsigned long)&kretprobe_trampoline;
    kprobe_opcode_t *correct_ret_addr = NULL;
+   void *frame_pointer;
+   bool skipped = false;

    INIT_HLIST_HEAD(&empty_rp);
    kretprobe_hash_lock(current, &head, &flags);
    /* fixup registers */
 #ifdef CONFIG_X86_64
    regs->cs = __KERNEL_CS;
+   /* On x86-64, we use pt_regs->sp for return address holder. */
+   frame_pointer = &regs->sp;
 #else
    regs->cs = __KERNEL_CS | get_kernel_rpl();
    regs->gs = 0;
+   /* On x86-32, we use pt_regs->flags for return address holder. */
+   frame_pointer = &regs->flags;
 #endif
    regs->ip = trampoline_address;
    regs->orig_ax = ~0UL;
@@ -789,8 +796,25 @@ static __used void *trampoline_handler(struct pt_regs *regs)
        if (ri->task != current)
            /* another task is sharing our hash bucket */
            continue;
+       /*
+        * Return probes must be pushed on this hash list correct
+        * order (same as return order) so that it can be poped
+        * correctly. However, if we find it is pushed it incorrect
+        * order, this means we find a function which should not be
+        * probed, because the wrong order entry is pushed on the
+        * path of processing other kretprobe itself.
+        */
+       if (ri->fp != frame_pointer) {
+           if (!skipped)
+               pr_warn("kretprobe is stacked incorrectly. Trying to fixup.\n");
+           skipped = true;
+           continue;
+       }

        orig_ret_address = (unsigned long)ri->ret_addr;
+       if (skipped)
+           pr_warn("%ps must be blacklisted because of incorrect kretprobe order\n",
+               ri->rp->kp.addr);

        if (orig_ret_address != trampoline_address)
            /*
@@ -808,6 +832,8 @@ static __used void *trampoline_handler(struct pt_regs *regs)
        if (ri->task != current)
            /* another task is sharing our hash bucket */
            continue;
+       if (ri->fp != frame_pointer)
+           continue;

        orig_ret_address = (unsigned long)ri->ret_addr;
        if (ri->rp && ri->rp->handler) {
diff --git a/include/linux/kprobes.h b/include/linux/kprobes.h
index 201f0f2..9a89725 100644
--- a/include/linux/kprobes.h
+++ b/include/linux/kprobes.h
@@ -173,6 +173,7 @@ struct kretprobe_instance {
    struct kretprobe *rp;
    kprobe_opcode_t *ret_addr;
    struct task_struct *task;
+   void *fp;
    char data[0];
 };

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