Re: [PATCH 1/2] ext4: Fix integer overflow in ext4fs_read_symlink()

[Heinrich Schuchardt]

On 12.07.24 13:14, Richard Weinberger wrote:

Am Freitag, 12. Juli 2024, 13:10:12 CEST schrieb 'Heinrich Schuchardt' via 
upstream:

On 02.07.24 21:42, Richard Weinberger wrote:

While zalloc() takes a size_t type, adding 1 to the le32 variable
will overflow.
A carefully crafted ext4 filesystem can exhibit an inode size of 0xffffffff
and as consequence zalloc() will do a zero allocation.

Later in the function the inode size is again used for copying data.
So an attacker can overwrite memory.

Avoid the overflow by using the __builtin_add_overflow() helper.

Signed-off-by: Richard Weinberger <rich...@nod.at>
---
   fs/ext4/ext4_common.c | 7 ++++++-
   1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/ext4_common.c b/fs/ext4/ext4_common.c
index 2ff0dca249..32364b72fb 100644
--- a/fs/ext4/ext4_common.c
+++ b/fs/ext4/ext4_common.c
@@ -2183,13 +2183,18 @@ static char *ext4fs_read_symlink(struct ext2fs_node 
*node)
        struct ext2fs_node *diro = node;
        int status;
        loff_t actread;
+       size_t alloc_size;

        if (!diro->inode_read) {
                status = ext4fs_read_inode(diro->data, diro->ino, &diro->inode);
                if (status == 0)
                        return NULL;
        }
-       symlink = zalloc(le32_to_cpu(diro->inode.size) + 1);
+
+       if (__builtin_add_overflow(le32_to_cpu(diro->inode.size), 1, 
&alloc_size))

U-Boot is freestanding code. You cannot use built-ins.

Hm, I see man built-ins in the U-Boot source.
Why is this one special?

See the definition of COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW in
include/linux/compiler-clang.h.

Best regards

Heinrich