Re: [PATCH v3] passt, util: Close any open file that the parent might have leaked

[Paul Holzinger <pholzing@redhat.com>]

On 07/08/2024 13:11, Stefano Brivio wrote:
> If a parent accidentally or due to implementation reasons leaks any
> open file, we don't want to have access to them, except for the file
> passed via --fd, if any.
>
> This is the case for Podman when Podman's parent leaks files into
> Podman: it's not practical for Podman to close unrelated files before
> starting pasta, as reported by Paul.
>
> Use close_range(2) to close all open files except for standard streams
> and the one from --fd.
>
> Given that parts of conf() depend on other files to be already opened,
> such as the epoll file descriptor, we can't easily defer this to a
> more convenient point, where --fd was already parsed. Introduce a
> minimal, duplicate version of --fd parsing to keep this simple.
>
> As we need to check that the passed --fd option doesn't exceed
> INT_MAX, because we'll parse it with strtol() but file descriptor
> indices are signed ints (regardless of the arguments close_range()
> take), extend the existing check in the actual --fd parsing in conf(),
> while at it.
>
> Suggested-by: Paul Holzinger <pholzing@redhat.com>
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
> v3: Handle --fd 3 case, and don't overflow if the --fd number exceeds
>      UINT_MAX: add an explicit check to ensure it's less than INT_MAX
>
> v2: Move call to close_open_files() to isolate_initial()
>
>   conf.c      |  3 ++-
>   isolation.c | 12 +++++++++---
>   isolation.h |  2 +-
>   passt.c     |  2 +-
>   util.c      | 38 ++++++++++++++++++++++++++++++++++++++
>   util.h      |  1 +
>   6 files changed, 52 insertions(+), 6 deletions(-)
>
> diff --git a/conf.c b/conf.c
> index 14d8ece..5422813 100644
> --- a/conf.c
> +++ b/conf.c
> @@ -1260,6 +1260,7 @@ void conf(struct ctx *c, int argc, char **argv)
>   	c->tcp.fwd_in.mode = c->tcp.fwd_out.mode = FWD_UNSET;
>   	c->udp.fwd_in.mode = c->udp.fwd_out.mode = FWD_UNSET;
>   
> +	optind = 1;
>   	do {
>   		name = getopt_long(argc, argv, optstring, options, NULL);
>   
> @@ -1426,7 +1427,7 @@ void conf(struct ctx *c, int argc, char **argv)
>   			errno = 0;
>   			c->fd_tap = strtol(optarg, NULL, 0);
>   
> -			if (c->fd_tap < 0 || errno)
> +			if (c->fd_tap < 0 || errno || c->fd_tap > INT_MAX)
>   				die("Invalid --fd: %s", optarg);
>   
>   			c->one_off = true;
> diff --git a/isolation.c b/isolation.c
> index 4956d7e..45fba1e 100644
> --- a/isolation.c
> +++ b/isolation.c
> @@ -29,7 +29,8 @@
>    *
>    * Executed immediately after startup, drops capabilities we don't
>    * need at any point during execution (or which we gain back when we
> - * need by joining other namespaces).
> + * need by joining other namespaces), and closes any leaked file we
> + * might have inherited from the parent process.
>    *
>    * 2. isolate_user()
>    * =================
> @@ -166,14 +167,17 @@ static void clamp_caps(void)
>   }
>   
>   /**
> - * isolate_initial() - Early, config independent self isolation
> + * isolate_initial() - Early, mostly config independent self isolation
> + * @argc:	Argument count
> + * @argv:	Command line options: only --fd (if present) is relevant here
>    *
>    * Should:
>    *  - drop unneeded capabilities
> + *  - close all open files except for standard streams and the one from --fd
>    * Musn't:
>    *  - remove filesytem access (we need to access files during setup)
>    */
> -void isolate_initial(void)
> +void isolate_initial(int argc, char **argv)
>   {
>   	uint64_t keep;
>   
> @@ -207,6 +211,8 @@ void isolate_initial(void)
>   		keep |= BIT(CAP_SETFCAP) | BIT(CAP_SYS_PTRACE);
>   
>   	drop_caps_ep_except(keep);
> +
> +	close_open_files(argc, argv);
>   }
>   
>   /**
> diff --git a/isolation.h b/isolation.h
> index 846b2af..80bb68d 100644
> --- a/isolation.h
> +++ b/isolation.h
> @@ -7,7 +7,7 @@
>   #ifndef ISOLATION_H
>   #define ISOLATION_H
>   
> -void isolate_initial(void);
> +void isolate_initial(int argc, char **argv);
>   void isolate_user(uid_t uid, gid_t gid, bool use_userns, const char *userns,
>   		  enum passt_modes mode);
>   int isolate_prefork(const struct ctx *c);
> diff --git a/passt.c b/passt.c
> index ea5bece..4b3c306 100644
> --- a/passt.c
> +++ b/passt.c
> @@ -211,7 +211,7 @@ int main(int argc, char **argv)
>   
>   	arch_avx2_exec(argv);
>   
> -	isolate_initial();
> +	isolate_initial(argc, argv);
>   
>   	c.pasta_netns_fd = c.fd_tap = c.pidfile_fd = -1;
>   
> diff --git a/util.c b/util.c
> index 07fb21c..9c6be6a 100644
> --- a/util.c
> +++ b/util.c
> @@ -26,6 +26,7 @@
>   #include <errno.h>
>   #include <stdbool.h>
>   #include <linux/errqueue.h>
> +#include <getopt.h>
>   
>   #include "util.h"
>   #include "iov.h"
> @@ -694,3 +695,40 @@ const char *str_ee_origin(const struct sock_extended_err *ee)
>   
>   	return "<invalid>";
>   }
> +
> +/**
> + * close_open_files() - Close leaked files, but not --fd, stdin, stdout, stderr
> + * @argc:	Argument count
> + * @argv:	Command line options, as we need to skip any file given via --fd
> + */
> +void close_open_files(int argc, char **argv)
> +{
> +	const struct option optfd[] = { { "fd", required_argument, NULL, 'F' },
> +					{ 0 },
> +				      };
> +	long fd = -1;
> +	int name;
> +
> +	do {
> +		name = getopt_long(argc, argv, ":F", optfd, NULL);
> +
> +		if (name == 'F') {
> +			errno = 0;
> +			fd = strtol(optarg, NULL, 0);
> +
> +			if (fd < 0 || errno || fd > INT_MAX)
> +				die("Invalid --fd: %s", optarg);
> +		}
> +	} while (name != -1);
> +
> +	if (fd == -1 || fd == 3) {
> +		unsigned int first = (fd == 3) ? 4 : 3;
> +
> +		if (close_range(first,	~0U,	CLOSE_RANGE_UNSHARE))
> +			die_perror("Failed to close files leaked by parent");
> +	} else {
> +		if (close_range(3,	fd - 1,	CLOSE_RANGE_UNSHARE) ||
> +		    close_range(fd + 1,	~0U,	CLOSE_RANGE_UNSHARE))
> +			die_perror("Failed to close files leaked by parent");
> +	}
Sorry that I didn't mentioned this before but doesn't this still fail 
when given fd 0, 1 or 2? I guess it should check if (fd < 3) in the 
die(Invalid --fd) case, unless someone sees a reason to allow passing 
the fd on stdio fds?
> +}
> diff --git a/util.h b/util.h
> index 83d2b53..cb4d181 100644
> --- a/util.h
> +++ b/util.h
> @@ -183,6 +183,7 @@ int __daemon(int pidfile_fd, int devnull_fd);
>   int fls(unsigned long x);
>   int write_file(const char *path, const char *buf);
>   int write_remainder(int fd, const struct iovec *iov, size_t iovcnt, size_t skip);
> +void close_open_files(int argc, char **argv);
>   
>   /**
>    * af_name() - Return name of an address family

-- 
Paul Holzinger