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    Subject[PATCH 5.4 59/77] ftrace: Fix NULL pointer dereference in is_ftrace_trampoline when ftrace is dead
    Date
    From: Yang Jihong <yangjihong1@huawei.com>

    commit c3b0f72e805f0801f05fa2aa52011c4bfc694c44 upstream.

    ftrace_startup does not remove ops from ftrace_ops_list when
    ftrace_startup_enable fails:

    register_ftrace_function
    ftrace_startup
    __register_ftrace_function
    ...
    add_ftrace_ops(&ftrace_ops_list, ops)
    ...
    ...
    ftrace_startup_enable // if ftrace failed to modify, ftrace_disabled is set to 1
    ...
    return 0 // ops is in the ftrace_ops_list.

    When ftrace_disabled = 1, unregister_ftrace_function simply returns without doing anything:
    unregister_ftrace_function
    ftrace_shutdown
    if (unlikely(ftrace_disabled))
    return -ENODEV; // return here, __unregister_ftrace_function is not executed,
    // as a result, ops is still in the ftrace_ops_list
    __unregister_ftrace_function
    ...

    If ops is dynamically allocated, it will be free later, in this case,
    is_ftrace_trampoline accesses NULL pointer:

    is_ftrace_trampoline
    ftrace_ops_trampoline
    do_for_each_ftrace_op(op, ftrace_ops_list) // OOPS! op may be NULL!

    Syzkaller reports as follows:
    [ 1203.506103] BUG: kernel NULL pointer dereference, address: 000000000000010b
    [ 1203.508039] #PF: supervisor read access in kernel mode
    [ 1203.508798] #PF: error_code(0x0000) - not-present page
    [ 1203.509558] PGD 800000011660b067 P4D 800000011660b067 PUD 130fb8067 PMD 0
    [ 1203.510560] Oops: 0000 [#1] SMP KASAN PTI
    [ 1203.511189] CPU: 6 PID: 29532 Comm: syz-executor.2 Tainted: G B W 5.10.0 #8
    [ 1203.512324] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
    [ 1203.513895] RIP: 0010:is_ftrace_trampoline+0x26/0xb0
    [ 1203.514644] Code: ff eb d3 90 41 55 41 54 49 89 fc 55 53 e8 f2 00 fd ff 48 8b 1d 3b 35 5d 03 e8 e6 00 fd ff 48 8d bb 90 00 00 00 e8 2a 81 26 00 <48> 8b ab 90 00 00 00 48 85 ed 74 1d e8 c9 00 fd ff 48 8d bb 98 00
    [ 1203.518838] RSP: 0018:ffffc900012cf960 EFLAGS: 00010246
    [ 1203.520092] RAX: 0000000000000000 RBX: 000000000000007b RCX: ffffffff8a331866
    [ 1203.521469] RDX: 0000000000000000 RSI: 0000000000000008 RDI: 000000000000010b
    [ 1203.522583] RBP: 0000000000000000 R08: 0000000000000000 R09: ffffffff8df18b07
    [ 1203.523550] R10: fffffbfff1be3160 R11: 0000000000000001 R12: 0000000000478399
    [ 1203.524596] R13: 0000000000000000 R14: ffff888145088000 R15: 0000000000000008
    [ 1203.525634] FS: 00007f429f5f4700(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
    [ 1203.526801] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    [ 1203.527626] CR2: 000000000000010b CR3: 0000000170e1e001 CR4: 00000000003706e0
    [ 1203.528611] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    [ 1203.529605] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

    Therefore, when ftrace_startup_enable fails, we need to rollback registration
    process and remove ops from ftrace_ops_list.

    Link: https://lkml.kernel.org/r/20220818032659.56209-1-yangjihong1@huawei.com

    Suggested-by: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/trace/ftrace.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    --- a/kernel/trace/ftrace.c
    +++ b/kernel/trace/ftrace.c
    @@ -2732,6 +2732,16 @@ int ftrace_startup(struct ftrace_ops *op

    ftrace_startup_enable(command);

    + /*
    + * If ftrace is in an undefined state, we just remove ops from list
    + * to prevent the NULL pointer, instead of totally rolling it back and
    + * free trampoline, because those actions could cause further damage.
    + */
    + if (unlikely(ftrace_disabled)) {
    + __unregister_ftrace_function(ops);
    + return -ENODEV;
    + }
    +
    ops->flags &= ~FTRACE_OPS_FL_ADDING;

    return 0;

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