Revision ee2e3f50629f17b0752b55b2566c15ce8dafb557 authored by Christian Brauner on 06 March 2021, 10:10:10 UTC, committed by Christian Brauner on 08 March 2021, 14:18:43 UTC
Creating a series of detached mounts, attaching them to the filesystem, and unmounting them can be used to trigger an integer overflow in ns->mounts causing the kernel to block any new mounts in count_mounts() and returning ENOSPC because it falsely assumes that the maximum number of mounts in the mount namespace has been reached, i.e. it thinks it can't fit the new mounts into the mount namespace anymore. Depending on the number of mounts in your system, this can be reproduced on any kernel that supportes open_tree() and move_mount() by compiling and running the following program: /* SPDX-License-Identifier: LGPL-2.1+ */ #define _GNU_SOURCE #include <errno.h> #include <fcntl.h> #include <getopt.h> #include <limits.h> #include <stdbool.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <sys/mount.h> #include <sys/stat.h> #include <sys/syscall.h> #include <sys/types.h> #include <unistd.h> /* open_tree() */ #ifndef OPEN_TREE_CLONE #define OPEN_TREE_CLONE 1 #endif #ifndef OPEN_TREE_CLOEXEC #define OPEN_TREE_CLOEXEC O_CLOEXEC #endif #ifndef __NR_open_tree #if defined __alpha__ #define __NR_open_tree 538 #elif defined _MIPS_SIM #if _MIPS_SIM == _MIPS_SIM_ABI32 /* o32 */ #define __NR_open_tree 4428 #endif #if _MIPS_SIM == _MIPS_SIM_NABI32 /* n32 */ #define __NR_open_tree 6428 #endif #if _MIPS_SIM == _MIPS_SIM_ABI64 /* n64 */ #define __NR_open_tree 5428 #endif #elif defined __ia64__ #define __NR_open_tree (428 + 1024) #else #define __NR_open_tree 428 #endif #endif /* move_mount() */ #ifndef MOVE_MOUNT_F_EMPTY_PATH #define MOVE_MOUNT_F_EMPTY_PATH 0x00000004 /* Empty from path permitted */ #endif #ifndef __NR_move_mount #if defined __alpha__ #define __NR_move_mount 539 #elif defined _MIPS_SIM #if _MIPS_SIM == _MIPS_SIM_ABI32 /* o32 */ #define __NR_move_mount 4429 #endif #if _MIPS_SIM == _MIPS_SIM_NABI32 /* n32 */ #define __NR_move_mount 6429 #endif #if _MIPS_SIM == _MIPS_SIM_ABI64 /* n64 */ #define __NR_move_mount 5429 #endif #elif defined __ia64__ #define __NR_move_mount (428 + 1024) #else #define __NR_move_mount 429 #endif #endif static inline int sys_open_tree(int dfd, const char *filename, unsigned int flags) { return syscall(__NR_open_tree, dfd, filename, flags); } static inline int sys_move_mount(int from_dfd, const char *from_pathname, int to_dfd, const char *to_pathname, unsigned int flags) { return syscall(__NR_move_mount, from_dfd, from_pathname, to_dfd, to_pathname, flags); } static bool is_shared_mountpoint(const char *path) { bool shared = false; FILE *f = NULL; char *line = NULL; int i; size_t len = 0; f = fopen("/proc/self/mountinfo", "re"); if (!f) return 0; while (getline(&line, &len, f) > 0) { char *slider1, *slider2; for (slider1 = line, i = 0; slider1 && i < 4; i++) slider1 = strchr(slider1 + 1, ' '); if (!slider1) continue; slider2 = strchr(slider1 + 1, ' '); if (!slider2) continue; *slider2 = '\0'; if (strcmp(slider1 + 1, path) == 0) { /* This is the path. Is it shared? */ slider1 = strchr(slider2 + 1, ' '); if (slider1 && strstr(slider1, "shared:")) { shared = true; break; } } } fclose(f); free(line); return shared; } static void usage(void) { const char *text = "mount-new [--recursive] <base-dir>\n"; fprintf(stderr, "%s", text); _exit(EXIT_SUCCESS); } #define exit_usage(format, ...) \ ({ \ fprintf(stderr, format "\n", ##__VA_ARGS__); \ usage(); \ }) #define exit_log(format, ...) \ ({ \ fprintf(stderr, format "\n", ##__VA_ARGS__); \ exit(EXIT_FAILURE); \ }) static const struct option longopts[] = { {"help", no_argument, 0, 'a'}, { NULL, no_argument, 0, 0 }, }; int main(int argc, char *argv[]) { int exit_code = EXIT_SUCCESS, index = 0; int dfd, fd_tree, new_argc, ret; char *base_dir; char *const *new_argv; char target[PATH_MAX]; while ((ret = getopt_long_only(argc, argv, "", longopts, &index)) != -1) { switch (ret) { case 'a': /* fallthrough */ default: usage(); } } new_argv = &argv[optind]; new_argc = argc - optind; if (new_argc < 1) exit_usage("Missing base directory\n"); base_dir = new_argv[0]; if (*base_dir != '/') exit_log("Please specify an absolute path"); /* Ensure that target is a shared mountpoint. */ if (!is_shared_mountpoint(base_dir)) exit_log("Please ensure that \"%s\" is a shared mountpoint", base_dir); dfd = open(base_dir, O_RDONLY | O_DIRECTORY | O_CLOEXEC); if (dfd < 0) exit_log("%m - Failed to open base directory \"%s\"", base_dir); ret = mkdirat(dfd, "detached-move-mount", 0755); if (ret < 0) exit_log("%m - Failed to create required temporary directories"); ret = snprintf(target, sizeof(target), "%s/detached-move-mount", base_dir); if (ret < 0 || (size_t)ret >= sizeof(target)) exit_log("%m - Failed to assemble target path"); /* * Having a mount table with 10000 mounts is already quite excessive * and shoult account even for weird test systems. */ for (size_t i = 0; i < 10000; i++) { fd_tree = sys_open_tree(dfd, "detached-move-mount", OPEN_TREE_CLONE | OPEN_TREE_CLOEXEC | AT_EMPTY_PATH); if (fd_tree < 0) { fprintf(stderr, "%m - Failed to open %d(detached-move-mount)", dfd); exit_code = EXIT_FAILURE; break; } ret = sys_move_mount(fd_tree, "", dfd, "detached-move-mount", MOVE_MOUNT_F_EMPTY_PATH); if (ret < 0) { if (errno == ENOSPC) fprintf(stderr, "%m - Buggy mount counting"); else fprintf(stderr, "%m - Failed to attach mount to %d(detached-move-mount)", dfd); exit_code = EXIT_FAILURE; break; } close(fd_tree); ret = umount2(target, MNT_DETACH); if (ret < 0) { fprintf(stderr, "%m - Failed to unmount %s", target); exit_code = EXIT_FAILURE; break; } } (void)unlinkat(dfd, "detached-move-mount", AT_REMOVEDIR); close(dfd); exit(exit_code); } and wait for the kernel to refuse any new mounts by returning ENOSPC. How many iterations are needed depends on the number of mounts in your system. Assuming you have something like 50 mounts on a standard system it should be almost instantaneous. The root cause of this is that detached mounts aren't handled correctly when source and target mount are identical and reside on a shared mount causing a broken mount tree where the detached source itself is propagated which propagation prevents for regular bind-mounts and new mounts. This ultimately leads to a miscalculation of the number of mounts in the mount namespace. Detached mounts created via open_tree(fd, path, OPEN_TREE_CLONE) are essentially like an unattached new mount, or an unattached bind-mount. They can then later on be attached to the filesystem via move_mount() which calls into attach_recursive_mount(). Part of attaching it to the filesystem is making sure that mounts get correctly propagated in case the destination mountpoint is MS_SHARED, i.e. is a shared mountpoint. This is done by calling into propagate_mnt() which walks the list of peers calling propagate_one() on each mount in this list making sure it receives the propagation event. The propagate_one() functions thereby skips both new mounts and bind mounts to not propagate them "into themselves". Both are identified by checking whether the mount is already attached to any mount namespace in mnt->mnt_ns. The is what the IS_MNT_NEW() helper is responsible for. However, detached mounts have an anonymous mount namespace attached to them stashed in mnt->mnt_ns which means that IS_MNT_NEW() doesn't realize they need to be skipped causing the mount to propagate "into itself" breaking the mount table and causing a disconnect between the number of mounts recorded as being beneath or reachable from the target mountpoint and the number of mounts actually recorded/counted in ns->mounts ultimately causing an overflow which in turn prevents any new mounts via the ENOSPC issue. So teach propagation to handle detached mounts by making it aware of them. I've been tracking this issue down for the last couple of days and then verifying that the fix is correct by unmounting everything in my current mount table leaving only /proc and /sys mounted and running the reproducer above overnight verifying the number of mounts counted in ns->mounts. With this fix the counts are correct and the ENOSPC issue can't be reproduced. This change will only have an effect on mounts created with the new mount API since detached mounts cannot be created with the old mount API so regressions are extremely unlikely. Link: https://lore.kernel.org/r/20210306101010.243666-1-christian.brauner@ubuntu.com Fixes: 2db154b3ea8e ("vfs: syscall: Add move_mount(2) to move mounts around") Cc: David Howells <dhowells@redhat.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Cc: linux-fsdevel@vger.kernel.org Cc: <stable@vger.kernel.org> Reviewed-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
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