[PATCH 1/2] exec: randomize the argument block's offset within its page

[Brian Witte <brianwitte@mailfence.com>]

bprm->p starts one word below STACK_TOP_MAX, and setup_arg_pages()
moves the stack only by whole pages, so the argv, envp and execfn
strings sit at the same offset within their page on every exec. Three
execs on x86_64 put argv[0] at 0x7ffeee30af9f, 0x7ffdf3709f9f and
0x7ffc871e0f9f.

Lower bprm->p by a random amount under a page before the strings are
copied: about 12 bits of entropy with 4K pages and 16 with 64K.
bprm->argmin is derived from bprm->p, so the argument size limit is
unchanged, but up to a page of RLIMIT_STACK is used.

ADDR_NO_RANDOMIZE disables the offset; PF_RANDOMIZE cannot be used,
since load_elf_binary() sets it after the strings are copied.
PER_CLEAR_ON_SETID also clears the flag for set-id execs only after the
copy, and moving the strings then would mean copying them all. So
bprm_fill_uid()'s set-id checks are repeated early, without its
inode_lock(), and a set-id exec gets the offset regardless of the flag.
A racing chmod or sysctl write can make the offset disagree with the ELF
loader but never affects the credentials. Only the file passed to
execve() is checked: set-id interpreters, file capabilities and LSM
transitions keep today's fixed offset under the flag. The checks need
bprm->file, so they run in bprm_mm_init(), not create_init_stack_vma().

The idea comes from PaX RANDUSTACK; linux-hardened carries a version by
Daniel Micay and Levente Polyak.

Suggested-by: Daniel Micay <danielmicay@gmail.com>
Link: https://github.com/KSPP/linux/issues/283
Signed-off-by: Brian Witte <brianwitte@mailfence.com>
---
 fs/exec.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 5d7b0fe32dbf..1070f0799b27 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -70,6 +70,7 @@
 #include <linux/user_events.h>
 #include <linux/rseq.h>
 #include <linux/ksm.h>
+#include <linux/random.h>
 
 #include <linux/uaccess.h>
 #include <asm/mmu_context.h>
@@ -195,6 +196,33 @@ static bool valid_arg_len(struct linux_binprm *bprm, long len)
 	return len <= MAX_ARG_STRLEN;
 }
 
+/*
+ * Called before credentials exist: treat ADDR_NO_RANDOMIZE as cleared for
+ * files bprm_fill_uid() will treat as set-id.
+ */
+static bool bprm_randomize_args(struct linux_binprm *bprm)
+{
+	struct file *file = bprm->file;
+	const struct mnt_idmap *idmap = file_mnt_idmap(file);
+	struct inode *inode = file_inode(file);
+	/* bprm->cred, not yet set, will be in the caller's namespace. */
+	struct user_namespace *ns = current_user_ns();
+	umode_t mode;
+
+	if (!READ_ONCE(randomize_va_space))
+		return false;
+	if (!(current->personality & ADDR_NO_RANDOMIZE))
+		return true;
+	if (!mnt_may_suid(file->f_path.mnt) || task_no_new_privs(current))
+		return false;
+	mode = READ_ONCE(inode->i_mode);
+	if (!(mode & S_ISUID) &&
+	    (mode & (S_ISGID | S_IXGRP)) != (S_ISGID | S_IXGRP))
+		return false;
+	return vfsuid_has_mapping(ns, i_uid_into_vfsuid(idmap, inode)) &&
+	       vfsgid_has_mapping(ns, i_gid_into_vfsgid(idmap, inode));
+}
+
 #else
 
 static inline void acct_arg_size(struct linux_binprm *bprm, unsigned long pages)
@@ -279,6 +307,10 @@ static int bprm_mm_init(struct linux_binprm *bprm)
 	err = create_init_stack_vma(bprm->mm, &bprm->vma, &bprm->p);
 	if (err)
 		goto err;
+
+	/* Randomize the strings' offset within their page; keep the top word. */
+	if (bprm_randomize_args(bprm))
+		bprm->p -= get_random_u32_below(PAGE_SIZE - sizeof(void *));
 #endif
 
 	return 0;
-- 
2.47.3