[v7,2/7] exec: Move S_ISREG() check earlier
diff mbox series

Message ID 20200723171227.446711-3-mic@digikod.net
State New
Headers show
Series
  • Add support for O_MAYEXEC
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Commit Message

Mickaël Salaün July 23, 2020, 5:12 p.m. UTC
From: Kees Cook <keescook@chromium.org>

The execve(2)/uselib(2) syscalls have always rejected non-regular
files. Recently, it was noticed that a deadlock was introduced when trying
to execute pipes, as the S_ISREG() test was happening too late. This was
fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
during execve()"), but it was added after inode_permission() had already
run, which meant LSMs could see bogus attempts to execute non-regular
files.

Move the test into the other inode type checks (which already look
for other pathological conditions[1]). Since there is no need to use
FMODE_EXEC while we still have access to "acc_mode", also switch the
test to MAY_EXEC.

Also include a comment with the redundant S_ISREG() checks at the end of
execve(2)/uselib(2) to note that they are present to avoid any mistakes.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
    struct open_flags open_exec_flags = {
        .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
        .acc_mode = MAY_EXEC,
        ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
                may_open(path, acc_mode, open_flag)
		    /* new location of MAY_EXEC vs S_ISREG() test */
                    inode_permission(inode, MAY_OPEN | acc_mode)
                        security_inode_permission(inode, acc_mode)
                vfs_open(path, file)
                    do_dentry_open(file, path->dentry->d_inode, open)
                        /* old location of FMODE_EXEC vs S_ISREG() test */
                        security_file_open(f)
                        open()

[1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/

Signed-off-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org
---
 fs/exec.c  | 14 ++++++++++++--
 fs/namei.c |  6 ++++--
 fs/open.c  |  6 ------
 3 files changed, 16 insertions(+), 10 deletions(-)

Comments

Eric W. Biederman Aug. 11, 2020, 7:27 p.m. UTC | #1
Mickaël Salaün <mic@digikod.net> writes:

> From: Kees Cook <keescook@chromium.org>
>
> The execve(2)/uselib(2) syscalls have always rejected non-regular
> files. Recently, it was noticed that a deadlock was introduced when trying
> to execute pipes, as the S_ISREG() test was happening too late. This was
> fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
> during execve()"), but it was added after inode_permission() had already
> run, which meant LSMs could see bogus attempts to execute non-regular
> files.
>
> Move the test into the other inode type checks (which already look
> for other pathological conditions[1]). Since there is no need to use
> FMODE_EXEC while we still have access to "acc_mode", also switch the
> test to MAY_EXEC.
>
> Also include a comment with the redundant S_ISREG() checks at the end of
> execve(2)/uselib(2) to note that they are present to avoid any mistakes.

The comment is:
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
Those comments scare me.  Why do you need to be extra cautious?
How can the file type possibly change between may_open and anywhere?
The type of a file is immutable after it's creation.

If the comment said check just in case something went wrong with
code maintenance I could understand but that isn't what the comment
says.

Also the fallthrough change below really should be broken out into
it's own change.


> My notes on the call path, and related arguments, checks, etc:
>
> do_open_execat()
>     struct open_flags open_exec_flags = {
>         .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
>         .acc_mode = MAY_EXEC,
>         ...
>     do_filp_open(dfd, filename, open_flags)
>         path_openat(nameidata, open_flags, flags)
>             file = alloc_empty_file(open_flags, current_cred());
>             do_open(nameidata, file, open_flags)
>                 may_open(path, acc_mode, open_flag)
> 		    /* new location of MAY_EXEC vs S_ISREG() test */
>                     inode_permission(inode, MAY_OPEN | acc_mode)
>                         security_inode_permission(inode, acc_mode)
>                 vfs_open(path, file)
>                     do_dentry_open(file, path->dentry->d_inode, open)
>                         /* old location of FMODE_EXEC vs S_ISREG() test */
>                         security_file_open(f)
>                         open()
>
> [1] https://lore.kernel.org/lkml/202006041910.9EF0C602@keescook/
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20200605160013.3954297-3-keescook@chromium.org
> ---
>  fs/exec.c  | 14 ++++++++++++--
>  fs/namei.c |  6 ++++--
>  fs/open.c  |  6 ------
>  3 files changed, 16 insertions(+), 10 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index d7c937044d10..bdc6a6eb5dce 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -141,8 +141,13 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
>  	if (IS_ERR(file))
>  		goto out;
>  
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
>  	error = -EACCES;
> -	if (!S_ISREG(file_inode(file)->i_mode))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>  		goto exit;
>  
>  	if (path_noexec(&file->f_path))
> @@ -886,8 +891,13 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
>  	if (IS_ERR(file))
>  		goto out;
>  
> +	/*
> +	 * may_open() has already checked for this, so it should be
> +	 * impossible to trip now. But we need to be extra cautious
> +	 * and check again at the very end too.
> +	 */
>  	err = -EACCES;
> -	if (!S_ISREG(file_inode(file)->i_mode))
> +	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
>  		goto exit;
>
>  	if (path_noexec(&file->f_path))
> diff --git a/fs/namei.c b/fs/namei.c
> index 72d4219c93ac..a559ad943970 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2849,16 +2849,18 @@ static int may_open(const struct path *path, int acc_mode, int flag)
>  	case S_IFLNK:
>  		return -ELOOP;
>  	case S_IFDIR:
> -		if (acc_mode & MAY_WRITE)
> +		if (acc_mode & (MAY_WRITE | MAY_EXEC))
>  			return -EISDIR;
>  		break;
>  	case S_IFBLK:
>  	case S_IFCHR:
>  		if (!may_open_dev(path))
>  			return -EACCES;
> -		/*FALLTHRU*/
> +		fallthrough;
                ^^^^^^^^^^^
That is an unrelated change and should be sent separately.

>  	case S_IFIFO:
>  	case S_IFSOCK:
> +		if (acc_mode & MAY_EXEC)
> +			return -EACCES;
>  		flag &= ~O_TRUNC;
>  		break;
>  	}
> diff --git a/fs/open.c b/fs/open.c
> index 6cd48a61cda3..623b7506a6db 100644
> --- a/fs/open.c
> +++ b/fs/open.c
> @@ -784,12 +784,6 @@ static int do_dentry_open(struct file *f,
>  		return 0;
>  	}
>  
> -	/* Any file opened for execve()/uselib() has to be a regular file. */
> -	if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) {
> -		error = -EACCES;
> -		goto cleanup_file;
> -	}
> -
>  	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
>  		error = get_write_access(inode);
>  		if (unlikely(error))

Patch
diff mbox series

diff --git a/fs/exec.c b/fs/exec.c
index d7c937044d10..bdc6a6eb5dce 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -141,8 +141,13 @@  SYSCALL_DEFINE1(uselib, const char __user *, library)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * may_open() has already checked for this, so it should be
+	 * impossible to trip now. But we need to be extra cautious
+	 * and check again at the very end too.
+	 */
 	error = -EACCES;
-	if (!S_ISREG(file_inode(file)->i_mode))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
 		goto exit;
 
 	if (path_noexec(&file->f_path))
@@ -886,8 +891,13 @@  static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * may_open() has already checked for this, so it should be
+	 * impossible to trip now. But we need to be extra cautious
+	 * and check again at the very end too.
+	 */
 	err = -EACCES;
-	if (!S_ISREG(file_inode(file)->i_mode))
+	if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
 		goto exit;
 
 	if (path_noexec(&file->f_path))
diff --git a/fs/namei.c b/fs/namei.c
index 72d4219c93ac..a559ad943970 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2849,16 +2849,18 @@  static int may_open(const struct path *path, int acc_mode, int flag)
 	case S_IFLNK:
 		return -ELOOP;
 	case S_IFDIR:
-		if (acc_mode & MAY_WRITE)
+		if (acc_mode & (MAY_WRITE | MAY_EXEC))
 			return -EISDIR;
 		break;
 	case S_IFBLK:
 	case S_IFCHR:
 		if (!may_open_dev(path))
 			return -EACCES;
-		/*FALLTHRU*/
+		fallthrough;
 	case S_IFIFO:
 	case S_IFSOCK:
+		if (acc_mode & MAY_EXEC)
+			return -EACCES;
 		flag &= ~O_TRUNC;
 		break;
 	}
diff --git a/fs/open.c b/fs/open.c
index 6cd48a61cda3..623b7506a6db 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -784,12 +784,6 @@  static int do_dentry_open(struct file *f,
 		return 0;
 	}
 
-	/* Any file opened for execve()/uselib() has to be a regular file. */
-	if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) {
-		error = -EACCES;
-		goto cleanup_file;
-	}
-
 	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = get_write_access(inode);
 		if (unlikely(error))