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    Subject[PATCH 5.15 298/846] x86/mce/inject: Avoid out-of-bounds write when setting flags
    Date
    From: Zhang Zixun <zhang133010@icloud.com>

    [ Upstream commit de768416b203ac84e02a757b782a32efb388476f ]

    A contrived zero-length write, for example, by using write(2):

    ...
    ret = write(fd, str, 0);
    ...

    to the "flags" file causes:

    BUG: KASAN: stack-out-of-bounds in flags_write
    Write of size 1 at addr ffff888019be7ddf by task writefile/3787

    CPU: 4 PID: 3787 Comm: writefile Not tainted 5.16.0-rc7+ #12
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-2 04/01/2014

    due to accessing buf one char before its start.

    Prevent such out-of-bounds access.

    [ bp: Productize into a proper patch. Link below is the next best
    thing because the original mail didn't get archived on lore. ]

    Fixes: 0451d14d0561 ("EDAC, mce_amd_inj: Modify flags attribute to use string arguments")
    Signed-off-by: Zhang Zixun <zhang133010@icloud.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Link: https://lore.kernel.org/linux-edac/YcnePfF1OOqoQwrX@zn.tnic/
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/x86/kernel/cpu/mce/inject.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/arch/x86/kernel/cpu/mce/inject.c b/arch/x86/kernel/cpu/mce/inject.c
    index 0bfc14041bbb4..b63b548497c14 100644
    --- a/arch/x86/kernel/cpu/mce/inject.c
    +++ b/arch/x86/kernel/cpu/mce/inject.c
    @@ -350,7 +350,7 @@ static ssize_t flags_write(struct file *filp, const char __user *ubuf,
    char buf[MAX_FLAG_OPT_SIZE], *__buf;
    int err;

    - if (cnt > MAX_FLAG_OPT_SIZE)
    + if (!cnt || cnt > MAX_FLAG_OPT_SIZE)
    return -EINVAL;

    if (copy_from_user(&buf, ubuf, cnt))
    --
    2.34.1


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