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    Subject[PATCH 6.0 450/862] arm64: ftrace: fix module PLTs with mcount
    Date
    From: Mark Rutland <mark.rutland@arm.com>

    [ Upstream commit 8cfb08575c6d4585f1ce0deeb189e5c824776b04 ]

    Li Huafei reports that mcount-based ftrace with module PLTs was broken
    by commit:

    a6253579977e4c6f ("arm64: ftrace: consistently handle PLTs.")

    When a module PLTs are used and a module is loaded sufficiently far away
    from the kernel, we'll create PLTs for any branches which are
    out-of-range. These are separate from the special ftrace trampoline
    PLTs, which the module PLT code doesn't directly manipulate.

    When mcount is in use this is a problem, as each mcount callsite in a
    module will be initialized to point to a module PLT, but since commit
    a6253579977e4c6f ftrace_make_nop() will assume that the callsite has
    been initialized to point to the special ftrace trampoline PLT, and
    ftrace_find_callable_addr() rejects other cases.

    This means that when ftrace tries to initialize a callsite via
    ftrace_make_nop(), the call to ftrace_find_callable_addr() will find
    that the `_mcount` stub is out-of-range and is not handled by the ftrace
    PLT, resulting in a splat:

    | ftrace_test: loading out-of-tree module taints kernel.
    | ftrace: no module PLT for _mcount
    | ------------[ ftrace bug ]------------
    | ftrace failed to modify
    | [<ffff800029180014>] 0xffff800029180014
    | actual: 44:00:00:94
    | Initializing ftrace call sites
    | ftrace record flags: 2000000
    | (0)
    | expected tramp: ffff80000802eb3c
    | ------------[ cut here ]------------
    | WARNING: CPU: 3 PID: 157 at kernel/trace/ftrace.c:2120 ftrace_bug+0x94/0x270
    | Modules linked in:
    | CPU: 3 PID: 157 Comm: insmod Tainted: G O 6.0.0-rc6-00151-gcd722513a189-dirty #22
    | Hardware name: linux,dummy-virt (DT)
    | pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
    | pc : ftrace_bug+0x94/0x270
    | lr : ftrace_bug+0x21c/0x270
    | sp : ffff80000b2bbaf0
    | x29: ffff80000b2bbaf0 x28: 0000000000000000 x27: ffff0000c4d38000
    | x26: 0000000000000001 x25: ffff800009d7e000 x24: ffff0000c4d86e00
    | x23: 0000000002000000 x22: ffff80000a62b000 x21: ffff8000098ebea8
    | x20: ffff0000c4d38000 x19: ffff80000aa24158 x18: ffffffffffffffff
    | x17: 0000000000000000 x16: 0a0d2d2d2d2d2d2d x15: ffff800009aa9118
    | x14: 0000000000000000 x13: 6333626532303830 x12: 3030303866666666
    | x11: 203a706d61727420 x10: 6465746365707865 x9 : 3362653230383030
    | x8 : c0000000ffffefff x7 : 0000000000017fe8 x6 : 000000000000bff4
    | x5 : 0000000000057fa8 x4 : 0000000000000000 x3 : 0000000000000001
    | x2 : ad2cb14bb5438900 x1 : 0000000000000000 x0 : 0000000000000022
    | Call trace:
    | ftrace_bug+0x94/0x270
    | ftrace_process_locs+0x308/0x430
    | ftrace_module_init+0x44/0x60
    | load_module+0x15b4/0x1ce8
    | __do_sys_init_module+0x1ec/0x238
    | __arm64_sys_init_module+0x24/0x30
    | invoke_syscall+0x54/0x118
    | el0_svc_common.constprop.4+0x84/0x100
    | do_el0_svc+0x3c/0xd0
    | el0_svc+0x1c/0x50
    | el0t_64_sync_handler+0x90/0xb8
    | el0t_64_sync+0x15c/0x160
    | ---[ end trace 0000000000000000 ]---
    | ---------test_init-----------

    Fix this by reverting to the old behaviour of ignoring the old
    instruction when initialising an mcount callsite in a module, which was
    the behaviour prior to commit a6253579977e4c6f.

    Signed-off-by: Mark Rutland <mark.rutland@arm.com>
    Fixes: a6253579977e ("arm64: ftrace: consistently handle PLTs.")
    Reported-by: Li Huafei <lihuafei1@huawei.com>
    Link: https://lore.kernel.org/linux-arm-kernel/20220929094134.99512-1-lihuafei1@huawei.com
    Cc: Ard Biesheuvel <ardb@kernel.org>
    Cc: Will Deacon <will@kernel.org>
    Link: https://lore.kernel.org/r/20220929134525.798593-1-mark.rutland@arm.com
    Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/arm64/kernel/ftrace.c | 17 ++++++++++++++++-
    1 file changed, 16 insertions(+), 1 deletion(-)

    diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
    index ea5dc7c90f46..b49ba9a24bcc 100644
    --- a/arch/arm64/kernel/ftrace.c
    +++ b/arch/arm64/kernel/ftrace.c
    @@ -217,11 +217,26 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
    unsigned long pc = rec->ip;
    u32 old = 0, new;

    + new = aarch64_insn_gen_nop();
    +
    + /*
    + * When using mcount, callsites in modules may have been initalized to
    + * call an arbitrary module PLT (which redirects to the _mcount stub)
    + * rather than the ftrace PLT we'll use at runtime (which redirects to
    + * the ftrace trampoline). We can ignore the old PLT when initializing
    + * the callsite.
    + *
    + * Note: 'mod' is only set at module load time.
    + */
    + if (!IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS) &&
    + IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) && mod) {
    + return aarch64_insn_patch_text_nosync((void *)pc, new);
    + }
    +
    if (!ftrace_find_callable_addr(rec, mod, &addr))
    return -EINVAL;

    old = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
    - new = aarch64_insn_gen_nop();

    return ftrace_modify_code(pc, old, new, true);
    }
    --
    2.35.1


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