| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <qztvfvesnxkaol6n3ucf5ovp2ssq4hzxceaedgfexieggzj6zh@pyd5f43pccyh>
Date: Mon, 12 Aug 2024 10:05:26 +0200
From: Alejandro Colomar <alx@...nel.org>
To: Yafang Shao <laoar.shao@...il.com>
Cc: akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
ebiederm@...ssion.com, alexei.starovoitov@...il.com, rostedt@...dmis.org,
catalin.marinas@....com, penguin-kernel@...ove.sakura.ne.jp, linux-mm@...ck.org,
linux-fsdevel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, audit@...r.kernel.org,
linux-security-module@...r.kernel.org, selinux@...r.kernel.org, bpf@...r.kernel.org,
netdev@...r.kernel.org, dri-devel@...ts.freedesktop.org,
Alexander Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
Kees Cook <keescook@...omium.org>, Matus Jokay <matus.jokay@...ba.sk>,
"Serge E. Hallyn" <serge@...lyn.com>
Subject: Re: [PATCH v6 1/9] Get rid of __get_task_comm()
Hi Yafang,
On Mon, Aug 12, 2024 at 10:29:25AM GMT, Yafang Shao 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.
>
> - 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.
>
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Link: https://lore.kernel.org/all/CAHk-=wivfrF0_zvf+oj6==Sh=-npJooP8chLPEfaFV0oNYTTBA@mail.gmail.com [0]
> Link: https://lore.kernel.org/all/CAHk-=whWtUC-AjmGJveAETKOMeMFSTwKwu99v7+b6AyHMmaDFA@mail.gmail.com/
> Suggested-by: Alejandro Colomar <alx@...nel.org>
> Link: https://lore.kernel.org/all/2jxak5v6dfxlpbxhpm3ey7oup4g2lnr3ueurfbosf5wdo65dk4@srb3hsk72zwq
> Signed-off-by: Yafang Shao <laoar.shao@...il.com>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: Christian Brauner <brauner@...nel.org>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Eric Biederman <ebiederm@...ssion.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Cc: Matus Jokay <matus.jokay@...ba.sk>
> Cc: Alejandro Colomar <alx@...nel.org>
> Cc: "Serge E. Hallyn" <serge@...lyn.com>
> ---
> fs/exec.c | 10 ----------
> fs/proc/array.c | 2 +-
> include/linux/sched.h | 31 +++++++++++++++++++++++++------
> kernel/kthread.c | 2 +-
> 4 files changed, 27 insertions(+), 18 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index a47d0e4c54f6..2e468ddd203a 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);
This comment is correct (see other comments below).
(Except that pedantically, I'd write it as NUL-terminated with a hyphen,
just like zero-padded.)
> - 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);
LGTM. (This would have been good even if not removing the helper.)
>
> 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 33dd8d9d2b85..e0e26edbda61 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1096,9 +1096,11 @@ 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
The comment above is inmprecise.
It should say either
"strscpy() to ensure it is always NUL-terminated", or
"strscpy_pad() to ensure it is NUL-terminated and zero-padded".
> + * - task_lock() to ensure the operation is atomic and the name is
> + * fully updated.
> */
> char comm[TASK_COMM_LEN];
>
> @@ -1912,10 +1914,27 @@ 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.
This comment has the same imprecission that I noted above.
> 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.
> + *
> + * - ARRAY_SIZE() can help ensure that @buf is indeed an array.
> + */
> #define get_task_comm(buf, tsk) ({ \
> - BUILD_BUG_ON(sizeof(buf) != TASK_COMM_LEN); \
> - __get_task_comm(buf, sizeof(buf), tsk); \
> + strscpy(buf, (tsk)->comm, ARRAY_SIZE(buf)); \
> + buf; \
> })
>
> #ifdef CONFIG_SMP
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index f7be976ff88a..7d001d033cf9 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -101,7 +101,7 @@ void get_kthread_comm(char *buf, size_t buf_size, struct task_struct *tsk)
> struct kthread *kthread = to_kthread(tsk);
>
> if (!kthread || !kthread->full_name) {
> - __get_task_comm(buf, buf_size, tsk);
> + strscpy(buf, tsk->comm, buf_size);
> return;
> }
Other than that, LGTM.
--
<https://www.alejandro-colomar.es/>
Download attachment "signature.asc" of type "application/pgp-signature" (834 bytes)
Powered by blists - more mailing lists