execve: Keep bprm->argmin behind CONFIG_MMU

When argmin was added in commit 655c16a8ce9c ("exec: separate
MM_ANONPAGES and RLIMIT_STACK accounting"), it was intended only for
validating stack limits on CONFIG_MMU[1]. All checking for reaching the
limit (argmin) is wrapped in CONFIG_MMU ifdef checks, though setting
argmin was not. That argmin is only supposed to be used under CONFIG_MMU
was rediscovered recently[2], and I don't want to trip over this again.

Move argmin's declaration into the existing CONFIG_MMU area, and add
helpers functions so the MMU tests can be consolidated.

Link: https://lore.kernel.org/all/20181126122307.GA1660@redhat.com
Link: https://lore.kernel.org/all/202406211253.7037F69@keescook/
Link: https://lore.kernel.org/r/20240621205046.4001362-1-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>

diff --git a/fs/exec.c b/fs/exec.c
index c3bec126505bad285e2ff720cadac110d3a59028..b7bc63bfb907a0226a67c681525bb2f53280fbdf 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -486,6 +486,23 @@ static int count_strings_kernel(const char *const *argv)
        return i;
 }
 
+static inline int bprm_set_stack_limit(struct linux_binprm *bprm,
+                                      unsigned long limit)
+{
+#ifdef CONFIG_MMU
+       bprm->argmin = bprm->p - limit;
+#endif
+       return 0;
+}
+static inline bool bprm_hit_stack_limit(struct linux_binprm *bprm)
+{
+#ifdef CONFIG_MMU
+       return bprm->p < bprm->argmin;
+#else
+       return false;
+#endif
+}
+
 /*
  * Calculate bprm->argmin from:
  * - _STK_LIM
@@ -532,8 +549,7 @@ static int bprm_stack_limits(struct linux_binprm *bprm)
                return -E2BIG;
        limit -= ptr_size;
 
-       bprm->argmin = bprm->p - limit;
-       return 0;
+       return bprm_set_stack_limit(bprm, limit);
 }
 
 /*
@@ -571,10 +587,8 @@ static int copy_strings(int argc, struct user_arg_ptr argv,
                pos = bprm->p;
                str += len;
                bprm->p -= len;
-#ifdef CONFIG_MMU
-               if (bprm->p < bprm->argmin)
+               if (bprm_hit_stack_limit(bprm))
                        goto out;
-#endif
 
                while (len > 0) {
                        int offset, bytes_to_copy;
@@ -649,7 +663,7 @@ int copy_string_kernel(const char *arg, struct linux_binprm *bprm)
        /* We're going to work our way backwards. */
        arg += len;
        bprm->p -= len;
-       if (IS_ENABLED(CONFIG_MMU) && bprm->p < bprm->argmin)
+       if (bprm_hit_stack_limit(bprm))
                return -E2BIG;
 
        while (len > 0) {

diff --git a/fs/exec_test.c b/fs/exec_test.c
index 32a90c6f47e79675219d6d308fddf90e2043118e..8fea0bf0b7f50974b0ed97083394f46f336a91e2 100644 (file)
--- a/fs/exec_test.c
+++ b/fs/exec_test.c
@@ -96,7 +96,9 @@ static void exec_test_bprm_stack_limits(struct kunit *test)
 
                rc = bprm_stack_limits(&bprm);
                KUNIT_EXPECT_EQ_MSG(test, rc, result->expected_rc, "on loop %d", i);
+#ifdef CONFIG_MMU
                KUNIT_EXPECT_EQ_MSG(test, bprm.argmin, result->expected_argmin, "on loop %d", i);
+#endif
        }
 }
 

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 70f97f685bffe62d38d2ff50d6e9c0208290adfb..e6c00e860951ae68f9ccda70992d2a43a7f13a7d 100644 (file)
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -19,13 +19,13 @@ struct linux_binprm {
 #ifdef CONFIG_MMU
        struct vm_area_struct *vma;
        unsigned long vma_pages;
+       unsigned long argmin; /* rlimit marker for copy_strings() */
 #else
 # define MAX_ARG_PAGES 32
        struct page *page[MAX_ARG_PAGES];
 #endif
        struct mm_struct *mm;
        unsigned long p; /* current top of mem */
-       unsigned long argmin; /* rlimit marker for copy_strings() */
        unsigned int
                /* Should an execfd be passed to userspace? */
                have_execfd:1,