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uprobes: Change uprobe_copy_process() to dup xol_area
This finally fixes the serious bug in uretprobes: a forked child crashes if the parent called fork() with the pending ret probe. Trivial test-case: # perf probe -x /lib/libc.so.6 __fork%return # perf record -e probe_libc:__fork perl -le 'fork || print "OK"' (the child doesn't print "OK", it is killed by SIGSEGV) If the child returns from the probed function it actually returns to trampoline_vaddr, because it got the copy of parent's stack mangled by prepare_uretprobe() when the parent entered this func. It crashes because a) this address is not mapped and b) until the previous change it doesn't have the proper->return_instances info. This means that uprobe_copy_process() has to create xol_area which has the trampoline slot, and its vaddr should be equal to parent's xol_area->vaddr. Unfortunately, uprobe_copy_process() can not simply do __create_xol_area(child, xol_area->vaddr). This could actually work but perf_event_mmap() doesn't expect the usage of foreign ->mm. So we offload this to task_work_run(), and pass the argument via not yet used utask->vaddr. We know that this vaddr is fine for install_special_mapping(), the necessary hole was recently "created" by dup_mmap() which skips the parent's VM_DONTCOPY area, and nobody else could use the new mm. Unfortunately, this also means that we can not handle the errors properly, we obviously can not abort the already completed fork(). So we simply print the warning if GFP_KERNEL allocation (the only possible reason) fails. Reported-by: Martin Cermak <mcermak@redhat.com> Reported-by: David Smith <dsmith@redhat.com> Signed-off-by: Oleg Nesterov <oleg@redhat.com> Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
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@ -35,6 +35,7 @@
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#include <linux/kdebug.h> /* notifier mechanism */
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#include "../../mm/internal.h" /* munlock_vma_page */
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#include <linux/percpu-rwsem.h>
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#include <linux/task_work.h>
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#include <linux/uprobes.h>
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@ -1400,6 +1401,17 @@ static void uprobe_warn(struct task_struct *t, const char *msg)
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current->comm, current->pid, msg);
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}
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static void dup_xol_work(struct callback_head *work)
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{
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kfree(work);
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if (current->flags & PF_EXITING)
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return;
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if (!__create_xol_area(current->utask->vaddr))
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uprobe_warn(current, "dup xol area");
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}
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/*
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* Called in context of a new clone/fork from copy_process.
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*/
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@ -1407,6 +1419,8 @@ void uprobe_copy_process(struct task_struct *t)
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{
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struct uprobe_task *utask = current->utask;
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struct mm_struct *mm = current->mm;
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struct callback_head *work;
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struct xol_area *area;
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t->utask = NULL;
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@ -1415,6 +1429,20 @@ void uprobe_copy_process(struct task_struct *t)
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if (dup_utask(t, utask))
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return uprobe_warn(t, "dup ret instances");
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/* The task can fork() after dup_xol_work() fails */
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area = mm->uprobes_state.xol_area;
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if (!area)
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return uprobe_warn(t, "dup xol area");
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/* TODO: move it into the union in uprobe_task */
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work = kmalloc(sizeof(*work), GFP_KERNEL);
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if (!work)
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return uprobe_warn(t, "dup xol area");
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utask->vaddr = area->vaddr;
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init_task_work(work, dup_xol_work);
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task_work_add(t, work, true);
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}
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/*
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