X-Git-Url: https://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2Fapply-default-acl.c;h=a4961957b7487fa10591f87e93ee54f41d3d2d31;hb=f0c1a9ee7c7a6acbfc0c4270f9167022504a6a45;hp=0f7e9ae91deb8c477c0fbae6b8161fc8d6fa2850;hpb=40f86500e755883aaa824dba16743bc89d693a42;p=apply-default-acl.git diff --git a/src/apply-default-acl.c b/src/apply-default-acl.c index 0f7e9ae..a496195 100644 --- a/src/apply-default-acl.c +++ b/src/apply-default-acl.c @@ -10,6 +10,7 @@ #define _GNU_SOURCE #include +#include /* AT_FOO constants */ #include /* nftw() et al. */ #include #include /* dirname() */ @@ -50,19 +51,44 @@ mode_t get_mode(const char* path) { } struct stat s; - int result = stat(path, &s); + int result = lstat(path, &s); if (result == 0) { return s.st_mode; } else { - /* errno will be set already by stat() */ + /* errno will be set already by lstat() */ return result; } } +/** + * @brief Determine if the given path might refer to an (unsafe) hard link. + * + * @param path + * The path to test. + * + * @return true if we are certain that @c path does not refer to a hard + * link, and false otherwise. In case of error, false is returned, + * because we are not sure that @c path is not a hard link. + */ +bool is_hardlink_safe(const char* path) { + if (path == NULL) { + return false; + } + struct stat s; + int result = lstat(path, &s); + if (result == 0) { + return (s.st_nlink == 1 || S_ISDIR(s.st_mode)); + } + else { + return false; + } +} + + /** * @brief Determine whether or not the given path is a regular file. * @@ -77,7 +103,7 @@ bool is_regular_file(const char* path) { } struct stat s; - int result = stat(path, &s); + int result = lstat(path, &s); if (result == 0) { return S_ISREG(s.st_mode); } @@ -88,6 +114,42 @@ bool is_regular_file(const char* path) { +/** + * @brief Determine whether or not the given path is accessible. + * + * @param path + * The path to test. + * + * @return true if @c path is accessible to the current effective + * user/group, false otherwise. + */ +bool path_accessible(const char* path) { + if (path == NULL) { + return false; + } + + /* Test for access using the effective user and group rather than + the real one. */ + int flags = AT_EACCESS; + + /* Don't follow symlinks when checking for a path's existence, + since we won't follow them to set its ACLs either. */ + flags |= AT_SYMLINK_NOFOLLOW; + + /* If the path is relative, interpret it relative to the current + working directory (just like the access() system call). */ + int result = faccessat(AT_FDCWD, path, F_OK, flags); + + if (result == 0) { + return true; + } + else { + return false; + } +} + + + /** * @brief Determine whether or not the given path is a directory. * @@ -102,7 +164,7 @@ bool is_directory(const char* path) { } struct stat s; - int result = stat(path, &s); + int result = lstat(path, &s); if (result == 0) { return S_ISDIR(s.st_mode); } @@ -494,59 +556,49 @@ int any_can_execute_or_dir(const char* path) { /** - * @brief Inherit the default ACL from @c parent to @c path. - * - * The @c parent parameter does not necessarily need to be the parent - * of @c path, although that will usually be the case. This overwrites - * any existing default ACL on @c path. + * @brief Set @c acl as the default ACL on @c path if it's a directory. * - * @param parent - * The parent directory whose ACL we want to inherit. + * This overwrites any existing default ACL on @c path. If no default + * ACL exists, then one is created. If @c path is not a directory, we + * return ACL_FAILURE but no error is raised. * * @param path - * The target directory whose ACL we wish to overwrite (or create). + * The target directory whose ACL we wish to replace or create. + * + * @param acl + * The ACL to set as default on @c path. * * @return - * - @c ACL_SUCCESS - The default ACL was inherited successfully. - * - @c ACL_FAILURE - Either @c parent or @c path is not a directory. + * - @c ACL_SUCCESS - The default ACL was assigned successfully. + * - @c ACL_FAILURE - If @c path is not a directory. * - @c ACL_ERROR - Unexpected library error. */ -int inherit_default_acl(const char* path, const char* parent) { - - /* Our return value. */ - int result = ACL_SUCCESS; +int assign_default_acl(const char* path, acl_t acl) { if (path == NULL) { errno = ENOENT; return ACL_ERROR; } - if (!is_directory(path) || !is_directory(parent)) { + if (!is_directory(path)) { return ACL_FAILURE; } - acl_t parent_acl = acl_get_file(parent, ACL_TYPE_DEFAULT); - if (parent_acl == (acl_t)NULL) { - perror("inherit_default_acl (acl_get_file)"); - return ACL_ERROR; - } - - acl_t path_acl = acl_dup(parent_acl); + /* Our return value; success unless something bad happens. */ + int result = ACL_SUCCESS; + acl_t path_acl = acl_dup(acl); if (path_acl == (acl_t)NULL) { perror("inherit_default_acl (acl_dup)"); - acl_free(parent_acl); - return ACL_ERROR; + return ACL_ERROR; /* Nothing to clean up in this case. */ } int sf_result = acl_set_file(path, ACL_TYPE_DEFAULT, path_acl); if (sf_result == -1) { perror("inherit_default_acl (acl_set_file)"); result = ACL_ERROR; - goto cleanup; } - cleanup: acl_free(path_acl); return result; } @@ -641,6 +693,32 @@ int apply_default_acl(const char* path, bool no_exec_mask) { return ACL_ERROR; } + /* Refuse to operate on hard links, which can be abused by an + * attacker to trick us into changing the ACL on a file we didn't + * intend to; namely the "target" of the hard link. To truly prevent + * that sort of mischief, we should be using file descriptors for + * the target and its parent directory. Then modulo a tiny race + * condition, we would be sure that "path" and "parent" don't change + * their nature between the time that we test them and when we + * utilize them. For contrast, the same attacker is free to replace + * "path" with a hard link after is_hardlink_safe() has returned + * "true" below. + * + * Unfortunately, our API is lacking in this area. For example, + * acl_set_fd() is only capable of setting the ACL_TYPE_ACCESS list, + * and not the ACL_TYPE_DEFAULT. Apparently the only way to operate + * on default ACLs is through the path name, which is inherently + * unreliable since the acl_*_file() calls themselves might follow + * links (both hard and symbolic). + * + * Some improvement could still be made by using descriptors where + * possible -- this would shrink the exploit window -- but for now + * we use a naive implementation that only keeps honest men honest. + */ + if (!is_hardlink_safe(path)) { + return ACL_FAILURE; + } + if (!is_regular_file(path) && !is_directory(path)) { return ACL_FAILURE; } @@ -698,7 +776,7 @@ int apply_default_acl(const char* path, bool no_exec_mask) { } /* If it's a directory, inherit the parent's default. */ - int inherit_result = inherit_default_acl(path, parent); + int inherit_result = assign_default_acl(path, defacl); if (inherit_result == ACL_ERROR) { perror("apply_default_acl (inherit_acls)"); result = ACL_ERROR; @@ -794,7 +872,7 @@ int apply_default_acl(const char* path, bool no_exec_mask) { * The program name to use in the output. * */ -void usage(char* program_name) { +void usage(const char* program_name) { printf("Apply any applicable default ACLs to the given files or " "directories.\n\n"); printf("Usage: %s [flags] [ [ ...]]\n\n", @@ -971,11 +1049,22 @@ int main(int argc, char* argv[]) { const char* target = argv[arg_index]; bool reapp_result = false; + /* Make sure we can access the given path before we go out of our + * way to please it. Doing this check outside of + * apply_default_acl() lets us spit out a better error message for + * typos, too. + */ + if (!path_accessible(target)) { + fprintf(stderr, "%s: %s: No such file or directory\n", argv[0], target); + result = EXIT_FAILURE; + continue; + } + if (recursive) { reapp_result = apply_default_acl_recursive(target, no_exec_mask); } else { - /* It's either normal file, or we're not operating recursively. */ + /* It's either a normal file, or we're not operating recursively. */ reapp_result = apply_default_acl(target, no_exec_mask); }