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    SubjectRe: [PATCH] arm64: fix oops in concurrently setting insn_emulation sysctls
    On Fri, Jul 01, 2022 at 02:27:17AM +0000, haibinzhang(张海斌) wrote:
    > How to reproduce:
    > launch two shell executions:
    > #!/bin/bash
    > while [ 1 ];
    > do
    > echo 1 > /proc/sys/abi/swp
    > done
    >
    > Oops info:
    > Unable to handle kernel NULL pointer dereference at virtual address 0000000000000010
    > Internal error: Oops: 96000006 [#1] SMP
    > Call trace:
    > update_insn_emulation_mode+0xc0/0x148
    > emulation_proc_handler+0x64/0xb8
    > proc_sys_call_handler+0x9c/0xf8
    > proc_sys_write+0x18/0x20
    > __vfs_write+0x20/0x48
    > vfs_write+0xe4/0x1d0
    > ksys_write+0x70/0xf8
    > __arm64_sys_write+0x20/0x28
    > el0_svc_common.constprop.0+0x7c/0x1c0
    > el0_svc_handler+0x2c/0xa0
    > el0_svc+0x8/0x200
    >
    > emulation_proc_handler changes table->data for proc_dointvec_minmax
    > and so it isn't allowed to reenter before restoring table->data,
    > which isn't right now.
    > To fix this issue, Add mutal exclusion covering related code section.

    typo: mutual

    > Signed-off-by: Haibin Zhang <haibinzhang@tencent.com>
    > ---
    > arch/arm64/kernel/armv8_deprecated.c | 9 +++++++--
    > 1 file changed, 7 insertions(+), 2 deletions(-)
    >
    > diff --git a/arch/arm64/kernel/armv8_deprecated.c b/arch/arm64/kernel/armv8_deprecated.c
    > index 6875a16..c519792 100644
    > --- a/arch/arm64/kernel/armv8_deprecated.c
    > +++ b/arch/arm64/kernel/armv8_deprecated.c
    > @@ -207,8 +207,12 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
    > loff_t *ppos)
    > {
    > int ret = 0;
    > - struct insn_emulation *insn = (struct insn_emulation *) table->data;
    > - enum insn_emulation_mode prev_mode = insn->current_mode;
    > + struct insn_emulation *insn;
    > + enum insn_emulation_mode prev_mode;
    > +
    > + raw_spin_lock(&insn_emulation_lock);
    > + insn = (struct insn_emulation *) table->data;
    > + prev_mode = insn->current_mode;
    >
    > table->data = &insn->current_mode;
    > ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
    > @@ -224,6 +228,7 @@ static int emulation_proc_handler(struct ctl_table *table, int write,
    > }
    > ret:
    > table->data = insn;
    > + raw_spin_unlock(&insn_emulation_lock);
    > return ret;

    This looks very similar to the patch previously posted here:

    https://lore.kernel.org/r/20220128090324.2727688-1-hewenliang4@huawei.com

    but Catalin's suggestion was ignored:

    https://lore.kernel.org/all/Yf0dxon1d07rzxZH@arm.com/

    Please can have you send a v2 along the line that he suggested?

    I also think a mutex is probably better than a spinlock given that we
    can end up cross-calling in the proc handler.
    Thanks,

    Will

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