Re: [PATCH] fs/exec.c: Add fast path for ENOENT on PATH search before allocating mm

[Kees Cook <keescook@chromium.org>]

On Fri, Sep 16, 2022 at 02:41:30PM +0100, Josh Triplett wrote:
> Currently, execve allocates an mm and parses argv and envp before
> checking if the path exists. However, the common case of a $PATH search
> may have several failed calls to exec before a single success. Do a
> filename lookup for the purposes of returning ENOENT before doing more
> expensive operations.
> 
> This does not create a TOCTTOU race, because this can only happen if the
> file didn't exist at some point during the exec call, and that point is
> permitted to be when we did our lookup.
> 
> To measure performance, I ran 2000 fork and execvpe calls with a
> seven-element PATH in which the file was found in the seventh directory
> (representative of the common case as /usr/bin is the seventh directory
> on my $PATH), as well as 2000 fork and execve calls with an absolute
> path to an existing binary. I recorded the minimum time for each, to
> eliminate noise from context switches and similar.
> 
> Without fast-path:
> fork/execvpe: 49876ns
> fork/execve:  32773ns
> 
> With fast-path:
> fork/execvpe: 36890ns
> fork/execve:  32069ns
> 
> The cost of the additional lookup seems to be in the noise for a
> successful exec, but it provides a 26% improvement for the path search
> case by speeding up the six failed execs.
> 
> Signed-off-by: Josh Triplett <josh@joshtriplett.org>
> ---
> 
> Discussed this at Plumbers with Kees Cook; turned out to be even more of
> a win than anticipated.
> 
>  fs/exec.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 9a5ca7b82bfc..fe786aeb2f1b 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1881,6 +1881,16 @@ static int do_execveat_common(int fd, struct filename *filename,
>  	if (IS_ERR(filename))
>  		return PTR_ERR(filename);
>  
> +	/* Fast-path ENOENT for $PATH search failures, before we alloc an mm or
> +	 * parse arguments. */
> +	if (fd == AT_FDCWD && flags == 0 && filename->name[0] == '/') {
> +		struct path path;
> +		retval = filename_lookup(AT_FDCWD, filename, 0, &path, NULL);
> +		if (retval == -ENOENT)

Oh, actually, I see the 0-day problem. This should be:

		if (retval < 0)

> +			goto out_ret;
> +		path_put(&path);

Otherwise this put will happen for an non-successful lookup that wans't
ENOENT. I've fixed this in my tree.

-Kees

> +	}
> +
>  	/*
>  	 * We move the actual failure in case of RLIMIT_NPROC excess from
>  	 * set*uid() to execve() because too many poorly written programs
> -- 
> 2.37.2
> 

-- 
Kees Cook