On 2020-04-07, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote:
> Some applications want to prevent the usual "lowest available fd"
> allocation from allocating certain file descriptors. For instance, they
> may want to prevent allocation of a closed fd 0, 1, or 2 other than via
> dup2/dup3, or reserve some low file descriptors for other purposes.
>
> Add a prctl to increase the minimum fd and return the previous minimum.
>
> System calls that allocate a specific file descriptor, such as
> dup2/dup3, ignore this minimum.
>
> exec resets the minimum fd, to prevent one program from interfering with
> another program's expectations about fd allocation.
Why is it implemented as an "increase the value" interface? It feels
like this is meant to avoid some kind of security trap (with a library
reducing the value) but it means that if you want to temporarily raise
the minimum fd number it's not possible (without re-exec()ing yourself,
which is hardly a fun thing to do).
Then again, this might've been discussed before and I missed it...
> Test program:
>
> #include <err.h>
> #include <fcntl.h>
> #include <stdio.h>
> #include <sys/prctl.h>
>
> int main(int argc, char *argv[])
> {
> if (prctl(PR_INCREASE_MIN_FD, 100, 0, 0, 0) < 0)
> err(1, "prctl");
> int fd = open("/dev/null", O_RDONLY);
> if (fd < 0)
> err(1, "open");
> printf("%d\n", fd); // prints 100
> return 0;
> }
>
> Signed-off-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> ---
> fs/file.c | 23 +++++++++++++++++------
> include/linux/fdtable.h | 1 +
> include/linux/file.h | 1 +
> include/uapi/linux/prctl.h | 3 +++
> kernel/sys.c | 5 +++++
> 5 files changed, 27 insertions(+), 6 deletions(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index c8a4e4c86e55..ba06140d89af 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -286,7 +286,6 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
> spin_lock_init(&newf->file_lock);
> newf->resize_in_progress = false;
> init_waitqueue_head(&newf->resize_wait);
> - newf->next_fd = 0;
> new_fdt = &newf->fdtab;
> new_fdt->max_fds = NR_OPEN_DEFAULT;
> new_fdt->close_on_exec = newf->close_on_exec_init;
> @@ -295,6 +294,7 @@ struct files_struct *dup_fd(struct files_struct *oldf, int *errorp)
> new_fdt->fd = &newf->fd_array[0];
>
> spin_lock(&oldf->file_lock);
> + newf->next_fd = newf->min_fd = oldf->min_fd;
> old_fdt = files_fdtable(oldf);
> open_files = count_open_files(old_fdt);
>
> @@ -487,9 +487,7 @@ int __alloc_fd(struct files_struct *files,
> spin_lock(&files->file_lock);
> repeat:
> fdt = files_fdtable(files);
> - fd = start;
> - if (fd < files->next_fd)
> - fd = files->next_fd;
> + fd = max3(start, files->min_fd, files->next_fd);
>
> if (fd < fdt->max_fds)
> fd = find_next_fd(fdt, fd);
> @@ -514,7 +512,7 @@ int __alloc_fd(struct files_struct *files,
> goto repeat;
>
> if (start <= files->next_fd)
> - files->next_fd = fd + 1;
> + files->next_fd = max(fd + 1, files->min_fd);
>
> __set_open_fd(fd, fdt);
> if (flags & O_CLOEXEC)
> @@ -555,7 +553,7 @@ static void __put_unused_fd(struct files_struct *files, unsigned int fd)
> {
> struct fdtable *fdt = files_fdtable(files);
> __clear_open_fd(fd, fdt);
> - if (fd < files->next_fd)
> + if (fd < files->next_fd && fd >= files->min_fd)
> files->next_fd = fd;
> }
>
> @@ -684,6 +682,7 @@ void do_close_on_exec(struct files_struct *files)
>
> /* exec unshares first */
> spin_lock(&files->file_lock);
> + files->min_fd = 0;
> for (i = 0; ; i++) {
> unsigned long set;
> unsigned fd = i * BITS_PER_LONG;
> @@ -865,6 +864,18 @@ bool get_close_on_exec(unsigned int fd)
> return res;
> }
>
> +unsigned int increase_min_fd(unsigned int num)
> +{
> + struct files_struct *files = current->files;
> + unsigned int old_min_fd;
> +
> + spin_lock(&files->file_lock);
> + old_min_fd = files->min_fd;
> + files->min_fd += num;
> + spin_unlock(&files->file_lock);
> + return old_min_fd;
> +}
> +
> static int do_dup2(struct files_struct *files,
> struct file *file, unsigned fd, unsigned flags)
> __releases(&files->file_lock)
> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index f07c55ea0c22..d1980443d8b3 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -60,6 +60,7 @@ struct files_struct {
> */
> spinlock_t file_lock ____cacheline_aligned_in_smp;
> unsigned int next_fd;
> + unsigned int min_fd; /* min for "lowest available fd" allocation */
> unsigned long close_on_exec_init[1];
> unsigned long open_fds_init[1];
> unsigned long full_fds_bits_init[1];
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 142d102f285e..b67986f818d2 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -88,6 +88,7 @@ extern bool get_close_on_exec(unsigned int fd);
> extern int __get_unused_fd_flags(unsigned flags, unsigned long nofile);
> extern int get_unused_fd_flags(unsigned flags);
> extern void put_unused_fd(unsigned int fd);
> +extern unsigned int increase_min_fd(unsigned int num);
>
> extern void fd_install(unsigned int fd, struct file *file);
>
> diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
> index 07b4f8131e36..916327272d21 100644
> --- a/include/uapi/linux/prctl.h
> +++ b/include/uapi/linux/prctl.h
> @@ -238,4 +238,7 @@ struct prctl_mm_map {
> #define PR_SET_IO_FLUSHER 57
> #define PR_GET_IO_FLUSHER 58
>
> +/* Increase minimum file descriptor for "lowest available fd" allocation */
> +#define PR_INCREASE_MIN_FD 59
> +
> #endif /* _LINUX_PRCTL_H */
> diff --git a/kernel/sys.c b/kernel/sys.c
> index d325f3ab624a..daa0ce43cecc 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -2514,6 +2514,11 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
>
> error = (current->flags & PR_IO_FLUSHER) == PR_IO_FLUSHER;
> break;
> + case PR_INCREASE_MIN_FD:
> + if (arg3 || arg4 || arg5)
> + return -EINVAL;
> + error = increase_min_fd((unsigned int)arg2);
> + break;
> default:
> error = -EINVAL;
> break;
> --
> 2.26.0
>
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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