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    SubjectRe: [PATCH v2 1/3] tracing: uprobes: Re-enable $comm support for uprobe events
    On Wed, 23 Jan 2019 03:40:05 -0500
    Steven Rostedt <rostedt@goodmis.org> wrote:

    > On Fri, 18 Jan 2019 13:44:25 +0900
    > Masami Hiramatsu <mhiramat@kernel.org> wrote:
    >
    > > Since commit 533059281ee5 ("tracing: probeevent: Introduce new
    > > argument fetching code") dropped the $comm support from uprobe
    > > events, this re-enables it.
    > >
    > > For $comm support, uses strlcpy() instead of strncpy_from_user()
    > > to copy current task's comm. Because it is in the kernel space,
    > > strncpy_from_user() always fails to copy the comm.
    > > This also uses strlen() instead of strnlen_user() to measure the
    > > length of the comm.
    > >
    > > Fixes: 533059281ee5 ("tracing: probeevent: Introduce new argument fetching code")
    > > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
    > > Reported-by: Andreas Ziegler <andreas.ziegler@fau.de>
    > > Acked-by: Andreas Ziegler <andreas.ziegler@fau.de>
    > > ---
    > > kernel/trace/trace_uprobe.c | 15 +++++++++++++--
    > > 1 file changed, 13 insertions(+), 2 deletions(-)
    > >
    > > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
    > > index 3a1d5ab6b4ba..b07e498ccbc6 100644
    > > --- a/kernel/trace/trace_uprobe.c
    > > +++ b/kernel/trace/trace_uprobe.c
    > > @@ -156,7 +156,10 @@ fetch_store_string(unsigned long addr, void *dest, void *base)
    > > if (unlikely(!maxlen))
    > > return -ENOMEM;
    > >
    > > - ret = strncpy_from_user(dst, src, maxlen);
    > > + if (addr == (unsigned long)current->comm)
    > > + ret = strlcpy(dst, current->comm, maxlen);
    >
    > As user space (although only root) defines the size of the event being
    > stored, and we could trick addr to be current->comm (although
    > difficult), we could possibly leak data if maxlen is > TASK_COMM_LEN. I
    > would feel better if we tested maxlen against TASK_COMM_LEN in this
    > case.
    >
    > if (maxlen > TASK_COMM_LEN)
    > maxlen = TASK_COMM_LEN;
    >
    > Or if we don't think it can happen, add a WARN_ON(maxlen >
    > TASK_COMM_LEN).

    Hmm, I thought current->comm is null terminated, isn't it?
    Anyway, if user can specify current->comm, he must be able to specify
    current->comm + TASK_COMM_LEN too by kprobe_events.
    Moreover, it can leak any data in kernel...

    And also, maxlen is calculated by fetch_store_strlen, right before
    this has been called.

    I rather concern the case that if we have shorter size of maxlen than
    current->comm. Would we better show "(fault)" or tail-cut name ?
    (of course this is very difficult to happen, since the length is
    already checked.)

    Thank you,

    >
    > -- Steve
    >
    >
    > > + else
    > > + ret = strncpy_from_user(dst, src, maxlen);
    > > if (ret >= 0) {
    > > if (ret == maxlen)
    > > dst[ret - 1] = '\0';
    > > @@ -180,7 +183,12 @@ fetch_store_strlen(unsigned long addr)
    > > int len;
    > > void __user *vaddr = (void __force __user *) addr;
    > >
    > > - len = strnlen_user(vaddr, MAX_STRING_SIZE);
    > > + if (addr == (unsigned long)current->comm) {
    > > + len = strlen(current->comm);
    > > + if (len)
    > > + len++;
    > > + } else
    > > + len = strnlen_user(vaddr, MAX_STRING_SIZE);
    > >
    > > return (len > MAX_STRING_SIZE) ? 0 : len;
    > > }
    > > @@ -220,6 +228,9 @@ process_fetch_insn(struct fetch_insn *code, struct pt_regs *regs, void *dest,
    > > case FETCH_OP_IMM:
    > > val = code->immediate;
    > > break;
    > > + case FETCH_OP_COMM:
    > > + val = (unsigned long)current->comm;
    > > + break;
    > > case FETCH_OP_FOFFS:
    > > val = translate_user_vaddr(code->immediate);
    > > break;
    >


    --
    Masami Hiramatsu <mhiramat@kernel.org>

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