X-Git-Url: https://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2Fapply-default-acl.c;h=54cf66a6ca523ce0f5d78252d9a0643dcf850ec0;hb=5fa3ee714935779d82594a88a252b154e7ee7a40;hp=19505282c2acda17960281e966a9d347a5bc1611;hpb=5a1f29cb6c306e9d8b19c4b3d964c4a05c0bc27d;p=apply-default-acl.git diff --git a/src/apply-default-acl.c b/src/apply-default-acl.c index 1950528..54cf66a 100644 --- a/src/apply-default-acl.c +++ b/src/apply-default-acl.c @@ -64,6 +64,31 @@ mode_t get_mode(const char* path) { +/** + * @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. * @@ -244,7 +269,13 @@ int acl_set_entry(acl_t* aclp, ACL. */ acl_entry_t new_entry; - /* We allocate memory here that we should release! */ + /* The acl_create_entry() function can allocate new memory and/or + * change the location of the ACL structure entirely. When that + * happens, the value pointed to by aclp is updated, which means + * that a new acl_t gets "passed out" to our caller, eventually to + * be fed to acl_free(). In other words, we should still be freeing + * the right thing, even if the value pointed to by aclp changes. + */ int c_result = acl_create_entry(aclp, &new_entry); if (c_result == ACL_ERROR) { perror("acl_set_entry (acl_create_entry)"); @@ -287,20 +318,20 @@ int acl_set_entry(acl_t* aclp, * @brief Determine the number of entries in the given ACL. * * @param acl - * A pointer to an @c acl_t structure. + * The ACL to inspect. * * @return Either the non-negative number of entries in @c acl, or * @c ACL_ERROR on error. */ -int acl_entry_count(acl_t* acl) { +int acl_entry_count(acl_t acl) { acl_entry_t entry; int entry_count = 0; - int result = acl_get_entry(*acl, ACL_FIRST_ENTRY, &entry); + int result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry); while (result == ACL_SUCCESS) { entry_count++; - result = acl_get_entry(*acl, ACL_NEXT_ENTRY, &entry); + result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry); } if (result == ACL_ERROR) { @@ -319,14 +350,14 @@ int acl_entry_count(acl_t* acl) { * An ACL is minimal if it has fewer than four entries. * * @param acl - * A pointer to an acl_t structure. + * The ACL whose minimality is in question. * * @return * - @c ACL_SUCCESS - @c acl is minimal * - @c ACL_FAILURE - @c acl is not minimal * - @c ACL_ERROR - Unexpected library error */ -int acl_is_minimal(acl_t* acl) { +int acl_is_minimal(acl_t acl) { int ec = acl_entry_count(acl); @@ -346,29 +377,17 @@ int acl_is_minimal(acl_t* acl) { /** - * @brief Determine whether the given path has an ACL whose mask - * denies execute. + * @brief Determine whether the given ACL's mask denies execute. * - * @param path - * The path to check. + * @param acl + * The ACL whose mask we want to check. * * @return - * - @c ACL_SUCCESS - @c path has a mask which denies execute. - * - @c ACL_FAILURE - The ACL for @c path does not deny execute, - * or @c path has no extended ACL at all. + * - @c ACL_SUCCESS - The @c acl has a mask which denies execute. + * - @c ACL_FAILURE - The @c acl has a mask which does not deny execute. * - @c ACL_ERROR - Unexpected library error. */ -int acl_execute_masked(const char* path) { - - acl_t acl = acl_get_file(path, ACL_TYPE_ACCESS); - - if (acl == (acl_t)NULL) { - perror("acl_execute_masked (acl_get_file)"); - return ACL_ERROR; - } - - /* Our return value. */ - int result = ACL_FAILURE; +int acl_execute_masked(acl_t acl) { acl_entry_t entry; int ge_result = acl_get_entry(acl, ACL_FIRST_ENTRY, &entry); @@ -379,8 +398,7 @@ int acl_execute_masked(const char* path) { if (tag_result == ACL_ERROR) { perror("acl_execute_masked (acl_get_tag_type)"); - result = ACL_ERROR; - goto cleanup; + return ACL_ERROR; } if (tag == ACL_MASK) { @@ -391,15 +409,13 @@ int acl_execute_masked(const char* path) { int ps_result = acl_get_permset(entry, &permset); if (ps_result == ACL_ERROR) { perror("acl_execute_masked (acl_get_permset)"); - result = ACL_ERROR; - goto cleanup; + return ACL_ERROR; } int gp_result = acl_get_perm(permset, ACL_EXECUTE); if (gp_result == ACL_ERROR) { perror("acl_execute_masked (acl_get_perm)"); - result = ACL_ERROR; - goto cleanup; + return ACL_ERROR; } if (gp_result == ACL_FAILURE) { @@ -411,9 +427,7 @@ int acl_execute_masked(const char* path) { ge_result = acl_get_entry(acl, ACL_NEXT_ENTRY, &entry); } - cleanup: - acl_free(acl); - return result; + return ACL_FAILURE; } @@ -457,7 +471,7 @@ int any_can_execute_or_dir(const char* path) { /* Our return value. */ int result = ACL_FAILURE; - if (acl_is_minimal(&acl)) { + if (acl_is_minimal(acl)) { mode_t mode = get_mode(path); if (mode & (S_IXUSR | S_IXOTH | S_IXGRP)) { result = ACL_SUCCESS; @@ -508,7 +522,7 @@ int any_can_execute_or_dir(const char* path) { if (gp_result == ACL_SUCCESS) { /* Only return ACL_SUCCESS if this execute bit is not masked. */ - if (acl_execute_masked(path) != ACL_SUCCESS) { + if (acl_execute_masked(acl) != ACL_SUCCESS) { result = ACL_SUCCESS; goto cleanup; } @@ -531,59 +545,49 @@ 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; } @@ -678,6 +682,32 @@ 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; } @@ -735,7 +765,7 @@ 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; @@ -791,7 +821,22 @@ int apply_default_acl(const char* path, bool no_exec_mask) { } } - /* Finally, add the permset to the access ACL. */ + /* Finally, add the permset to the access ACL. It's actually + * important that we pass in the address of "acl" here, and not + * "acl" itself. Why? The call to acl_create_entry() within + * acl_set_entry() can allocate new memory for the entry. + * Sometimes that can be done in-place, in which case everything + * is cool and the new memory gets released when we call + * acl_free(acl). + * + * But occasionally, the whole ACL structure will have to be moved + * in order to allocate the extra space. When that happens, + * acl_create_entry() modifies the pointer it was passed (in this + * case, &acl) to point to the new location. We want to call + * acl_free() on the new location, and since acl_free() gets + * called right here, we need acl_create_entry() to update the + * value of "acl". To do that, it needs the address of "acl". + */ int set_result = acl_set_entry(&acl, entry); if (set_result == ACL_ERROR) { perror("apply_default_acl (acl_set_entry)");