The comment above uprobe_write_opcode() is wrong, unapply_uprobe() calls
it under mmap_read_lock() and this is correct.
And it is completely unclear why register_for_each_vma() takes mmap_lock
for writing, add a comment to explain that mmap_write_lock() is needed to
avoid the following race:

        - A task T hits the bp installed by uprobe and calls
          find_active_uprobe()

        - uprobe_unregister() removes this uprobe/bp

        - T calls find_uprobe() which returns NULL

        - another uprobe_register() installs the bp at the same address

        - T calls is_trap_at_addr() which returns true

        - T returns to handle_swbp() and gets SIGTRAP.

Reported-by: Andrii Nakryiko <and...@kernel.org>
Signed-off-by: Oleg Nesterov <o...@redhat.com>
Acked-by: Andrii Nakryiko <and...@kernel.org>
Acked-by: Masami Hiramatsu (Google) <mhira...@kernel.org>
Reviewed-by: Jiri Olsa <jo...@kernel.org>
---
 kernel/events/uprobes.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 73cc47708679..73dd12b09a7b 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -453,7 +453,7 @@ static int update_ref_ctr(struct uprobe *uprobe, struct 
mm_struct *mm,
  * @vaddr: the virtual address to store the opcode.
  * @opcode: opcode to be written at @vaddr.
  *
- * Called with mm->mmap_lock held for write.
+ * Called with mm->mmap_lock held for read or write.
  * Return 0 (success) or a negative errno.
  */
 int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
@@ -1046,7 +1046,13 @@ register_for_each_vma(struct uprobe *uprobe, struct 
uprobe_consumer *new)
 
                if (err && is_register)
                        goto free;
-
+               /*
+                * We take mmap_lock for writing to avoid the race with
+                * find_active_uprobe() which takes mmap_lock for reading.
+                * Thus this install_breakpoint() can not make
+                * is_trap_at_addr() true right after find_uprobe()
+                * returns NULL in find_active_uprobe().
+                */
                mmap_write_lock(mm);
                vma = find_vma(mm, info->vaddr);
                if (!vma || !valid_vma(vma, is_register) ||
-- 
2.25.1.362.g51ebf55


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