Revert "fs: don't block i_writecount during exec"

This reverts commit 2a010c4128.

Rui Ueyama <rui314@gmail.com> writes:

> I'm the creator and the maintainer of the mold linker
> (https://github.com/rui314/mold). Recently, we discovered that mold
> started causing process crashes in certain situations due to a change
> in the Linux kernel. Here are the details:
>
> - In general, overwriting an existing file is much faster than
> creating an empty file and writing to it on Linux, so mold attempts to
> reuse an existing executable file if it exists.
>
> - If a program is running, opening the executable file for writing
> previously failed with ETXTBSY. If that happens, mold falls back to
> creating a new file.
>
> - However, the Linux kernel recently changed the behavior so that
> writing to an executable file is now always permitted
> (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=2a010c412853).
>
> That caused mold to write to an executable file even if there's a
> process running that file. Since changes to mmap'ed files are
> immediately visible to other processes, any processes running that
> file would almost certainly crash in a very mysterious way.
> Identifying the cause of these random crashes took us a few days.
>
> Rejecting writes to an executable file that is currently running is a
> well-known behavior, and Linux had operated that way for a very long
> time. So, I don’t believe relying on this behavior was our mistake;
> rather, I see this as a regression in the Linux kernel.

Quoting myself from commit 2a010c4128 ("fs: don't block i_writecount during exec")

> Yes, someone in userspace could potentially be relying on this. It's not
> completely out of the realm of possibility but let's find out if that's
> actually the case and not guess.

It seems we found out that someone is relying on this obscure behavior.
So revert the change.

Link: https://github.com/rui314/mold/issues/1361
Link: https://lore.kernel.org/r/4a2bc207-76be-4715-8e12-7fc45a76a125@leemhuis.info
Cc: <stable@vger.kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/binfmt_elf.c

@@ -1257,6 +1257,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		}
 		reloc_func_desc = interp_load_addr;
 
+		allow_write_access(interpreter);
 		fput(interpreter);
 
 		kfree(interp_elf_ex);
@@ -1353,6 +1354,7 @@ static int load_elf_binary(struct linux_binprm *bprm)
 	kfree(interp_elf_ex);
 	kfree(interp_elf_phdata);
 out_free_file:
+	allow_write_access(interpreter);
 	if (interpreter)
 		fput(interpreter);
 out_free_ph:

fs/binfmt_elf_fdpic.c

@@ -394,6 +394,7 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 			goto error;
 		}
 
+		allow_write_access(interpreter);
 		fput(interpreter);
 		interpreter = NULL;
 	}
@@ -465,8 +466,10 @@ static int load_elf_fdpic_binary(struct linux_binprm *bprm)
 	retval = 0;
 
 error:
-	if (interpreter)
+	if (interpreter) {
+		allow_write_access(interpreter);
 		fput(interpreter);
+	}
 	kfree(interpreter_name);
 	kfree(exec_params.phdrs);
 	kfree(exec_params.loadmap);

fs/binfmt_misc.c

@@ -247,10 +247,13 @@ static int load_misc_binary(struct linux_binprm *bprm)
 	if (retval < 0)
 		goto ret;
 
-	if (fmt->flags & MISC_FMT_OPEN_FILE)
+	if (fmt->flags & MISC_FMT_OPEN_FILE) {
 		interp_file = file_clone_open(fmt->interp_file);
-	else
+		if (!IS_ERR(interp_file))
+			deny_write_access(interp_file);
+	} else {
 		interp_file = open_exec(fmt->interpreter);
+	}
 	retval = PTR_ERR(interp_file);
 	if (IS_ERR(interp_file))
 		goto ret;

fs/exec.c

@@ -883,7 +883,8 @@ EXPORT_SYMBOL(transfer_args_to_stack);
  */
 static struct file *do_open_execat(int fd, struct filename *name, int flags)
 {
-	struct file *file;
+	int err;
+	struct file *file __free(fput) = NULL;
 	struct open_flags open_exec_flags = {
 		.open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
 		.acc_mode = MAY_EXEC,
@@ -908,12 +909,14 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	 * an invariant that all non-regular files error out before we get here.
 	 */
 	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)) ||
-	    path_noexec(&file->f_path)) {
-		fput(file);
+	    path_noexec(&file->f_path))
 		return ERR_PTR(-EACCES);
-	}
 
-	return file;
+	err = deny_write_access(file);
+	if (err)
+		return ERR_PTR(err);
+
+	return no_free_ptr(file);
 }
 
 /**
@@ -923,7 +926,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
  *
  * Returns ERR_PTR on failure or allocated struct file on success.
  *
- * As this is a wrapper for the internal do_open_execat(). Also see
+ * As this is a wrapper for the internal do_open_execat(), callers
+ * must call allow_write_access() before fput() on release. Also see
  * do_close_execat().
  */
 struct file *open_exec(const char *name)
@@ -1465,8 +1469,10 @@ static int prepare_bprm_creds(struct linux_binprm *bprm)
 /* Matches do_open_execat() */
 static void do_close_execat(struct file *file)
 {
-	if (file)
-		fput(file);
+	if (!file)
+		return;
+	allow_write_access(file);
+	fput(file);
 }
 
 static void free_bprm(struct linux_binprm *bprm)
@@ -1791,6 +1797,7 @@ static int exec_binprm(struct linux_binprm *bprm)
 		bprm->file = bprm->interpreter;
 		bprm->interpreter = NULL;
 
+		allow_write_access(exec);
 		if (unlikely(bprm->have_execfd)) {
 			if (bprm->executable) {
 				fput(exec);

kernel/fork.c

@@ -621,6 +621,12 @@ static void dup_mm_exe_file(struct mm_struct *mm, struct mm_struct *oldmm)
 
 	exe_file = get_mm_exe_file(oldmm);
 	RCU_INIT_POINTER(mm->exe_file, exe_file);
+	/*
+	 * We depend on the oldmm having properly denied write access to the
+	 * exe_file already.
+	 */
+	if (exe_file && deny_write_access(exe_file))
+		pr_warn_once("deny_write_access() failed in %s\n", __func__);
 }
 
 #ifdef CONFIG_MMU
@@ -1413,11 +1419,20 @@ int set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 	 */
 	old_exe_file = rcu_dereference_raw(mm->exe_file);
 
-	if (new_exe_file)
+	if (new_exe_file) {
+		/*
+		 * We expect the caller (i.e., sys_execve) to already denied
+		 * write access, so this is unlikely to fail.
+		 */
+		if (unlikely(deny_write_access(new_exe_file)))
+			return -EACCES;
 		get_file(new_exe_file);
+	}
 	rcu_assign_pointer(mm->exe_file, new_exe_file);
-	if (old_exe_file)
+	if (old_exe_file) {
+		allow_write_access(old_exe_file);
 		fput(old_exe_file);
+	}
 	return 0;
 }
 
@@ -1456,6 +1471,9 @@ int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 			return ret;
 	}
 
+	ret = deny_write_access(new_exe_file);
+	if (ret)
+		return -EACCES;
 	get_file(new_exe_file);
 
 	/* set the new file */
@@ -1464,8 +1482,10 @@ int replace_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file)
 	rcu_assign_pointer(mm->exe_file, new_exe_file);
 	mmap_write_unlock(mm);
 
-	if (old_exe_file)
+	if (old_exe_file) {
+		allow_write_access(old_exe_file);
 		fput(old_exe_file);
+	}
 	return 0;
 }