From 0e64363bb21a07fa318017ea8c90597db63a9545 Mon Sep 17 00:00:00 2001 From: David Tardon Date: Thu, 3 Jan 2019 14:37:34 +0100 Subject: [PATCH] fs-util: add new CHASE_SAFE flag to chase_symlinks() When the flag is specified we won't transition to a privilege-owned file or directory from an unprivileged-owned one. This is useful when privileged code wants to load data from a file unprivileged users have write access to, and validates the ownership, but want's to make sure that no symlink games are played to read a root-owned system file belonging to a different context. (cherry picked from commit f14f1806e329fe92d01f15c22a384702f0cb4ae0) Related: #1663143 --- src/shared/util.c | 43 +++++++++++++++++++++++++++++++++++++++++++ src/shared/util.h | 7 ++++--- src/test/test-util.c | 26 ++++++++++++++++++++++++++ 3 files changed, 73 insertions(+), 3 deletions(-) diff --git a/src/shared/util.c b/src/shared/util.c index 385551f2b3..fc4887920f 100644 --- a/src/shared/util.c +++ b/src/shared/util.c @@ -9203,10 +9203,22 @@ int fd_is_fs_type(int fd, statfs_f_type_t magic_value) { return is_fs_type(&s, magic_value); } +static bool safe_transition(const struct stat *a, const struct stat *b) { + /* Returns true if the transition from a to b is safe, i.e. that we never transition from unprivileged to + * privileged files or directories. Why bother? So that unprivileged code can't symlink to privileged files + * making us believe we read something safe even though it isn't safe in the specific context we open it in. */ + + if (a->st_uid == 0) /* Transitioning from privileged to unprivileged is always fine */ + return true; + + return a->st_uid == b->st_uid; /* Otherwise we need to stay within the same UID */ +} + int chase_symlinks(const char *path, const char *original_root, unsigned flags, char **ret) { _cleanup_free_ char *buffer = NULL, *done = NULL, *root = NULL; _cleanup_close_ int fd = -1; unsigned max_follow = 32; /* how many symlinks to follow before giving up and returning ELOOP */ + struct stat previous_stat; bool exists = true; char *todo; int r; @@ -9250,6 +9262,11 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags, if (fd < 0) return -errno; + if (flags & CHASE_SAFE) { + if (fstat(fd, &previous_stat) < 0) + return -errno; + } + todo = buffer; for (;;) { _cleanup_free_ char *first = NULL; @@ -9313,6 +9330,16 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags, if (fd_parent < 0) return -errno; + if (flags & CHASE_SAFE) { + if (fstat(fd_parent, &st) < 0) + return -errno; + + if (!safe_transition(&previous_stat, &st)) + return -EPERM; + + previous_stat = st; + } + safe_close(fd); fd = fd_parent; @@ -9347,6 +9374,12 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags, if (fstat(child, &st) < 0) return -errno; + if ((flags & CHASE_SAFE) && + !safe_transition(&previous_stat, &st)) + return -EPERM; + + previous_stat = st; + if ((flags & CHASE_NO_AUTOFS) && fd_is_fs_type(child, AUTOFS_SUPER_MAGIC) > 0) return -EREMOTE; @@ -9379,6 +9412,16 @@ int chase_symlinks(const char *path, const char *original_root, unsigned flags, free(done); + if (flags & CHASE_SAFE) { + if (fstat(fd, &st) < 0) + return -errno; + + if (!safe_transition(&previous_stat, &st)) + return -EPERM; + + previous_stat = st; + } + /* Note that we do not revalidate the root, we take it as is. */ if (isempty(root)) done = NULL; diff --git a/src/shared/util.h b/src/shared/util.h index 915c7439e8..fa3e2e3009 100644 --- a/src/shared/util.h +++ b/src/shared/util.h @@ -1157,9 +1157,10 @@ bool is_fs_type(const struct statfs *s, statfs_f_type_t magic_value) _pure_; int fd_is_fs_type(int fd, statfs_f_type_t magic_value); enum { - CHASE_PREFIX_ROOT = 1, /* If set, the specified path will be prefixed by the specified root before beginning the iteration */ - CHASE_NONEXISTENT = 2, /* If set, it's OK if the path doesn't actually exist. */ - CHASE_NO_AUTOFS = 4, /* If set, return -EREMOTE if autofs mount point found */ + CHASE_PREFIX_ROOT = 1U << 0, /* If set, the specified path will be prefixed by the specified root before beginning the iteration */ + CHASE_NONEXISTENT = 1U << 1, /* If set, it's OK if the path doesn't actually exist. */ + CHASE_NO_AUTOFS = 1U << 2, /* If set, return -EREMOTE if autofs mount point found */ + CHASE_SAFE = 1U << 3, /* If set, return EPERM if we ever traverse from unprivileged to privileged files or directories */ }; int chase_symlinks(const char *path_with_prefix, const char *root, unsigned flags, char **ret); diff --git a/src/test/test-util.c b/src/test/test-util.c index 397c45a9f4..e5a646ec20 100644 --- a/src/test/test-util.c +++ b/src/test/test-util.c @@ -2113,6 +2113,32 @@ static void test_chase_symlinks(void) { r = chase_symlinks(p, NULL, 0, &result); assert_se(r == -ENOENT); + if (geteuid() == 0) { + p = strjoina(temp, "/priv1"); + assert_se(mkdir(p, 0755) >= 0); + + q = strjoina(p, "/priv2"); + assert_se(mkdir(q, 0755) >= 0); + + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) >= 0); + + assert_se(chown(q, 65534, 65534) >= 0); + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) >= 0); + + assert_se(chown(p, 65534, 65534) >= 0); + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) >= 0); + + assert_se(chown(q, 0, 0) >= 0); + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) == -EPERM); + + assert_se(rmdir(q) >= 0); + assert_se(symlink("/etc/passwd", q) >= 0); + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) == -EPERM); + + assert_se(chown(p, 0, 0) >= 0); + assert_se(chase_symlinks(q, NULL, CHASE_SAFE, NULL) >= 0); + } + assert_se(rm_rf_dangerous(temp, false, true, false) >= 0); }