uprobes: Fix race in uprobe_free_utask

JIRA: https://issues.redhat.com/browse/RHEL-120479

Conflicts: changed context due to missing upstream commit 430af82
           ("uprobes: kill the unnecessary put_uprobe/xol_free_insn_slot
           in uprobe_free_utask()").

commit b583ef8
Author: Jiri Olsa <jolsa@kernel.org>
Date:   Thu Jan 9 15:14:40 2025 +0100

    uprobes: Fix race in uprobe_free_utask

    Max Makarov reported kernel panic [1] in perf user callchain code.

    The reason for that is the race between uprobe_free_utask and bpf
    profiler code doing the perf user stack unwind and is triggered
    within uprobe_free_utask function:
      - after current->utask is freed and
      - before current->utask is set to NULL

     general protection fault, probably for non-canonical address 0x9e759c37ee555c76: 0000 [#1] SMP PTI
     RIP: 0010:is_uprobe_at_func_entry+0x28/0x80
     ...
      ? die_addr+0x36/0x90
      ? exc_general_protection+0x217/0x420
      ? asm_exc_general_protection+0x26/0x30
      ? is_uprobe_at_func_entry+0x28/0x80
      perf_callchain_user+0x20a/0x360
      get_perf_callchain+0x147/0x1d0
      bpf_get_stackid+0x60/0x90
      bpf_prog_9aac297fb833e2f5_do_perf_event+0x434/0x53b
      ? __smp_call_single_queue+0xad/0x120
      bpf_overflow_handler+0x75/0x110
      ...
      asm_sysvec_apic_timer_interrupt+0x1a/0x20
     RIP: 0010:__kmem_cache_free+0x1cb/0x350
     ...
      ? uprobe_free_utask+0x62/0x80
      ? acct_collect+0x4c/0x220
      uprobe_free_utask+0x62/0x80
      mm_release+0x12/0xb0
      do_exit+0x26b/0xaa0
      __x64_sys_exit+0x1b/0x20
      do_syscall_64+0x5a/0x80

    It can be easily reproduced by running following commands in
    separate terminals:

      # while :; do bpftrace -e 'uprobe:/bin/ls:_start  { printf("hit\n"); }' -c ls; done
      # bpftrace -e 'profile:hz:100000 { @[ustack()] = count(); }'

    Fixing this by making sure current->utask pointer is set to NULL
    before we start to release the utask object.

    [1] grafana/pyroscope#3673

    Fixes: cfa7f3d ("perf,x86: avoid missing caller address in stack traces captured in uprobe")
    Reported-by: Max Makarov <maxpain@linux.com>
    Signed-off-by: Jiri Olsa <jolsa@kernel.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Acked-by: Oleg Nesterov <oleg@redhat.com>
    Acked-by: Andrii Nakryiko <andrii@kernel.org>
    Link: https://lore.kernel.org/r/20250109141440.2692173-1-jolsa@kernel.org

Signed-off-by: Viktor Malik <vmalik@redhat.com>

Author: viktormalik

kernel/events/uprobes.c

@@ -1720,6 +1720,7 @@ void uprobe_free_utask(struct task_struct *t)
 	if (!utask)
 		return;
 
+	t->utask = NULL;
 	if (utask->active_uprobe)
 		put_uprobe(utask->active_uprobe);
 
@@ -1729,7 +1730,6 @@ void uprobe_free_utask(struct task_struct *t)
 
 	xol_free_insn_slot(t);
 	kfree(utask);
-	t->utask = NULL;
 }
 
 /*