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