On Wed, Aug 28, 2024 at 10:04 PM Kees Cook <[email protected]> wrote:
>
>
>
> On August 27, 2024 8:03:14 PM PDT, Yafang Shao <[email protected]> wrote:
> >We want to eliminate the use of __get_task_comm() for the following
> >reasons:
> >
> >- The task_lock() is unnecessary
> > Quoted from Linus [0]:
> > : Since user space can randomly change their names anyway, using locking
> > : was always wrong for readers (for writers it probably does make sense
> > : to have some lock - although practically speaking nobody cares there
> > : either, but at least for a writer some kind of race could have
> > : long-term mixed results
> >
> >- The BUILD_BUG_ON() doesn't add any value
> > The only requirement is to ensure that the destination buffer is a valid
> > array.
>
> Sorry, that's not a correct evaluation. See below.
>
> >
> >- Zeroing is not necessary in current use cases
> > To avoid confusion, we should remove it. Moreover, not zeroing could
> > potentially make it easier to uncover bugs. If the caller needs a
> > zero-padded task name, it should be explicitly handled at the call site.
>
> This is also not an appropriate rationale. We don't make the kernel "more
> buggy" not purpose. ;) See below.
>
> >
> >Suggested-by: Linus Torvalds <[email protected]>
> >Link:
> >https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npjoop8chlpefafv0onyt...@mail.gmail.com
> > [0]
> >Link:
> >https://lore.kernel.org/all/CAHk-=whwtuc-ajmgjveaetkomemfstwkwu99v7+b6ayhmma...@mail.gmail.com/
> >Suggested-by: Alejandro Colomar <[email protected]>
> >Link:
> >https://lore.kernel.org/all/2jxak5v6dfxlpbxhpm3ey7oup4g2lnr3ueurfbosf5wdo65dk4@srb3hsk72zwq
> >Signed-off-by: Yafang Shao <[email protected]>
> >Cc: Alexander Viro <[email protected]>
> >Cc: Christian Brauner <[email protected]>
> >Cc: Jan Kara <[email protected]>
> >Cc: Eric Biederman <[email protected]>
> >Cc: Kees Cook <[email protected]>
> >Cc: Alexei Starovoitov <[email protected]>
> >Cc: Matus Jokay <[email protected]>
> >Cc: Alejandro Colomar <[email protected]>
> >Cc: "Serge E. Hallyn" <[email protected]>
> >---
> > fs/exec.c | 10 ----------
> > fs/proc/array.c | 2 +-
> > include/linux/sched.h | 32 ++++++++++++++++++++++++++------
> > kernel/kthread.c | 2 +-
> > 4 files changed, 28 insertions(+), 18 deletions(-)
> >
> >diff --git a/fs/exec.c b/fs/exec.c
> >index 50e76cc633c4..8a23171bc3c3 100644
> >--- a/fs/exec.c
> >+++ b/fs/exec.c
> >@@ -1264,16 +1264,6 @@ static int unshare_sighand(struct task_struct *me)
> > return 0;
> > }
> >
> >-char *__get_task_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> >-{
> >- task_lock(tsk);
> >- /* Always NUL terminated and zero-padded */
> >- strscpy_pad(buf, tsk->comm, buf_size);
> >- task_unlock(tsk);
> >- return buf;
> >-}
> >-EXPORT_SYMBOL_GPL(__get_task_comm);
> >-
> > /*
> > * These functions flushes out all traces of the currently running
> > executable
> > * so that a new one can be started
> >diff --git a/fs/proc/array.c b/fs/proc/array.c
> >index 34a47fb0c57f..55ed3510d2bb 100644
> >--- a/fs/proc/array.c
> >+++ b/fs/proc/array.c
> >@@ -109,7 +109,7 @@ void proc_task_name(struct seq_file *m, struct
> >task_struct *p, bool escape)
> > else if (p->flags & PF_KTHREAD)
> > get_kthread_comm(tcomm, sizeof(tcomm), p);
> > else
> >- __get_task_comm(tcomm, sizeof(tcomm), p);
> >+ get_task_comm(tcomm, p);
> >
> > if (escape)
> > seq_escape_str(m, tcomm, ESCAPE_SPACE | ESCAPE_SPECIAL,
> > "\n\\");
> >diff --git a/include/linux/sched.h b/include/linux/sched.h
> >index f8d150343d42..c40b95a79d80 100644
> >--- a/include/linux/sched.h
> >+++ b/include/linux/sched.h
> >@@ -1096,9 +1096,12 @@ struct task_struct {
> > /*
> > * executable name, excluding path.
> > *
> >- * - normally initialized setup_new_exec()
> >- * - access it with [gs]et_task_comm()
> >- * - lock it with task_lock()
> >+ * - normally initialized begin_new_exec()
> >+ * - set it with set_task_comm()
> >+ * - strscpy_pad() to ensure it is always NUL-terminated and
> >+ * zero-padded
> >+ * - task_lock() to ensure the operation is atomic and the name is
> >+ * fully updated.
> > */
> > char comm[TASK_COMM_LEN];
> >
> >@@ -1914,10 +1917,27 @@ static inline void set_task_comm(struct task_struct
> >*tsk, const char *from)
> > __set_task_comm(tsk, from, false);
> > }
> >
> >-extern char *__get_task_comm(char *to, size_t len, struct task_struct *tsk);
> >+/*
> >+ * - Why not use task_lock()?
> >+ * User space can randomly change their names anyway, so locking for
> >readers
> >+ * doesn't make sense. For writers, locking is probably necessary, as a
> >race
> >+ * condition could lead to long-term mixed results.
> >+ * The strscpy_pad() in __set_task_comm() can ensure that the task comm is
> >+ * always NUL-terminated and zero-padded. Therefore the race condition
> >between
> >+ * reader and writer is not an issue.
> >+ *
> >+ * - Why not use strscpy_pad()?
> >+ * While strscpy_pad() prevents writing garbage past the NUL terminator,
> >which
> >+ * is useful when using the task name as a key in a hash map, most use
> >cases
> >+ * don't require this. Zero-padding might confuse users if it’s
> >unnecessary,
> >+ * and not zeroing might even make it easier to expose bugs. If you need a
> >+ * zero-padded task name, please handle that explicitly at the call site.
>
> I really don't like this part of the change. You don't know that existing
> callers don't depend on the padding. Please invert this logic:
> get_task_comm() must use strscpy_pad(). Calls NOT wanting padding can call
> strscpy() themselves.
>
> >+ *
> >+ * - ARRAY_SIZE() can help ensure that @buf is indeed an array.
>
> This doesn't need checking here; strscpy() will already do that.
>
> >+ */
> > #define get_task_comm(buf, tsk) ({ \
> >- BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN); \
>
> Also, please leave the TASK_COMM_LEN test so that destination buffers
> continue to be the correct size: current callers do not perform any return
> value analysis, so they cannot accidentally start having situations where the
> destination string might be truncated. Again, anyone wanting to avoid that
> restriction can use strscpy() directly and check the return value.
Hello Kees,
Thanks for your input.
Alejandro has addressed all the other changes except for the removal
of BUILD_BUG_ON(). I have a question regarding this: if we're using it
to avoid truncation, why not write it like this?
BUILD_BUG_ON(sizeof(buf) < TASK_COMM_LEN);
This way, it ensures that the size is at least as large as TASK_COMM_LEN.
--
Regards
Yafang
