Re: [PATCH v2 1/6] fs/readdir: Fix filldir() and filldir64() use of user_access_begin()

[Linus Torvalds <torvalds@linux-foundation.org>]

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[ Talking to myself ]

On Wed, Jan 22, 2020 at 12:00 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> COMPLETELY UNTESTED! It compiles for me. The generated assembly looks
> ok from a quick look.

Some more testing shows that objtool is unhappy about how we do that
signal_pending(current) inside the user access region.

I didn't notice because my test builds were with sane kernel
configurations so that I could look at the generated code.

But with KASAN enabled, the signal check causes accesses that KASAN
wants to check, and I get

  objtool: filldir()+0x395: call to __kasan_check_read() with UACCESS enabled

warnings.

So that patch of mine isn't acceptable for silly reasons, and the
signal check itself would need to be done outside of the user access
area.

That actually makes the whole "let's do the &prev->d_off setting
unconditionally" much more interesting.

So here's a slightly updated patch that does exactly that, and avoids
the objtool warning.

It actually generates better code than the last one too, because now
we don't duplicate the user_access_end() for the EINTR case.

So test this one instead, please.

                 Linus

[-- Attachment #2: patch.diff --]
[-- Type: text/x-patch, Size: 5997 bytes --]

 fs/readdir.c | 70 +++++++++++++++++++++++++++++-------------------------------
 1 file changed, 34 insertions(+), 36 deletions(-)

diff --git a/fs/readdir.c b/fs/readdir.c
index d26d5ea4de7b..35be4ca6b354 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -206,7 +206,7 @@ struct linux_dirent {
 struct getdents_callback {
 	struct dir_context ctx;
 	struct linux_dirent __user * current_dir;
-	struct linux_dirent __user * previous;
+	int prev_reclen;
 	int count;
 	int error;
 };
@@ -214,12 +214,13 @@ struct getdents_callback {
 static int filldir(struct dir_context *ctx, const char *name, int namlen,
 		   loff_t offset, u64 ino, unsigned int d_type)
 {
-	struct linux_dirent __user * dirent;
+	struct linux_dirent __user *dirent, *prev;
 	struct getdents_callback *buf =
 		container_of(ctx, struct getdents_callback, ctx);
 	unsigned long d_ino;
 	int reclen = ALIGN(offsetof(struct linux_dirent, d_name) + namlen + 2,
 		sizeof(long));
+	int prev_reclen;
 
 	buf->error = verify_dirent_name(name, namlen);
 	if (unlikely(buf->error))
@@ -232,28 +233,24 @@ static int filldir(struct dir_context *ctx, const char *name, int namlen,
 		buf->error = -EOVERFLOW;
 		return -EOVERFLOW;
 	}
-	dirent = buf->previous;
-	if (dirent && signal_pending(current))
+	prev_reclen = buf->prev_reclen;
+	if (prev_reclen && signal_pending(current))
 		return -EINTR;
-
-	/*
-	 * Note! This range-checks 'previous' (which may be NULL).
-	 * The real range was checked in getdents
-	 */
-	if (!user_access_begin(dirent, sizeof(*dirent)))
-		goto efault;
-	if (dirent)
-		unsafe_put_user(offset, &dirent->d_off, efault_end);
 	dirent = buf->current_dir;
+	prev = (void __user *) dirent - prev_reclen;
+	if (!user_access_begin(prev, reclen + prev_reclen))
+		goto efault;
+
+	/* This might be 'dirent->d_off', but if so it will get overwritten */
+	unsafe_put_user(offset, &prev->d_off, efault_end);
 	unsafe_put_user(d_ino, &dirent->d_ino, efault_end);
 	unsafe_put_user(reclen, &dirent->d_reclen, efault_end);
 	unsafe_put_user(d_type, (char __user *) dirent + reclen - 1, efault_end);
 	unsafe_copy_dirent_name(dirent->d_name, name, namlen, efault_end);
 	user_access_end();
 
-	buf->previous = dirent;
-	dirent = (void __user *)dirent + reclen;
-	buf->current_dir = dirent;
+	buf->current_dir = (void __user *)dirent + reclen;
+	buf->prev_reclen = reclen;
 	buf->count -= reclen;
 	return 0;
 efault_end:
@@ -267,7 +264,6 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd,
 		struct linux_dirent __user *, dirent, unsigned int, count)
 {
 	struct fd f;
-	struct linux_dirent __user * lastdirent;
 	struct getdents_callback buf = {
 		.ctx.actor = filldir,
 		.count = count,
@@ -285,8 +281,10 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd,
 	error = iterate_dir(f.file, &buf.ctx);
 	if (error >= 0)
 		error = buf.error;
-	lastdirent = buf.previous;
-	if (lastdirent) {
+	if (buf.prev_reclen) {
+		struct linux_dirent __user * lastdirent;
+		lastdirent = (void __user *)buf.current_dir - buf.prev_reclen;
+
 		if (put_user(buf.ctx.pos, &lastdirent->d_off))
 			error = -EFAULT;
 		else
@@ -299,7 +297,7 @@ SYSCALL_DEFINE3(getdents, unsigned int, fd,
 struct getdents_callback64 {
 	struct dir_context ctx;
 	struct linux_dirent64 __user * current_dir;
-	struct linux_dirent64 __user * previous;
+	int prev_reclen;
 	int count;
 	int error;
 };
@@ -307,11 +305,12 @@ struct getdents_callback64 {
 static int filldir64(struct dir_context *ctx, const char *name, int namlen,
 		     loff_t offset, u64 ino, unsigned int d_type)
 {
-	struct linux_dirent64 __user *dirent;
+	struct linux_dirent64 __user *dirent, *prev;
 	struct getdents_callback64 *buf =
 		container_of(ctx, struct getdents_callback64, ctx);
 	int reclen = ALIGN(offsetof(struct linux_dirent64, d_name) + namlen + 1,
 		sizeof(u64));
+	int prev_reclen;
 
 	buf->error = verify_dirent_name(name, namlen);
 	if (unlikely(buf->error))
@@ -319,30 +318,28 @@ static int filldir64(struct dir_context *ctx, const char *name, int namlen,
 	buf->error = -EINVAL;	/* only used if we fail.. */
 	if (reclen > buf->count)
 		return -EINVAL;
-	dirent = buf->previous;
-	if (dirent && signal_pending(current))
+	prev_reclen = buf->prev_reclen;
+	if (prev_reclen && signal_pending(current))
 		return -EINTR;
-
-	/*
-	 * Note! This range-checks 'previous' (which may be NULL).
-	 * The real range was checked in getdents
-	 */
-	if (!user_access_begin(dirent, sizeof(*dirent)))
-		goto efault;
-	if (dirent)
-		unsafe_put_user(offset, &dirent->d_off, efault_end);
 	dirent = buf->current_dir;
+	prev = (void __user *)dirent - prev_reclen;
+	if (!user_access_begin(prev, reclen + prev_reclen))
+		goto efault;
+
+	/* This might be 'dirent->d_off', but if so it will get overwritten */
+	unsafe_put_user(offset, &prev->d_off, efault_end);
 	unsafe_put_user(ino, &dirent->d_ino, efault_end);
 	unsafe_put_user(reclen, &dirent->d_reclen, efault_end);
 	unsafe_put_user(d_type, &dirent->d_type, efault_end);
 	unsafe_copy_dirent_name(dirent->d_name, name, namlen, efault_end);
 	user_access_end();
 
-	buf->previous = dirent;
+	buf->prev_reclen = reclen;
 	dirent = (void __user *)dirent + reclen;
 	buf->current_dir = dirent;
 	buf->count -= reclen;
 	return 0;
+
 efault_end:
 	user_access_end();
 efault:
@@ -354,7 +351,6 @@ int ksys_getdents64(unsigned int fd, struct linux_dirent64 __user *dirent,
 		    unsigned int count)
 {
 	struct fd f;
-	struct linux_dirent64 __user * lastdirent;
 	struct getdents_callback64 buf = {
 		.ctx.actor = filldir64,
 		.count = count,
@@ -372,9 +368,11 @@ int ksys_getdents64(unsigned int fd, struct linux_dirent64 __user *dirent,
 	error = iterate_dir(f.file, &buf.ctx);
 	if (error >= 0)
 		error = buf.error;
-	lastdirent = buf.previous;
-	if (lastdirent) {
+	if (buf.prev_reclen) {
+		struct linux_dirent64 __user * lastdirent;
 		typeof(lastdirent->d_off) d_off = buf.ctx.pos;
+
+		lastdirent = (void __user *) buf.current_dir - buf.prev_reclen;
 		if (__put_user(d_off, &lastdirent->d_off))
 			error = -EFAULT;
 		else