exfat: fix improper check of dentry.stream.valid_size

We found an infinite loop bug in the exFAT file system that can lead to a
Denial-of-Service (DoS) condition. When a dentry in an exFAT filesystem is
malformed, the following system calls — SYS_openat, SYS_ftruncate, and
SYS_pwrite64 — can cause the kernel to hang.

Root cause analysis shows that the size validation code in exfat_find()
does not check whether dentry.stream.valid_size is negative. As a result,
the system calls mentioned above can succeed and eventually trigger the DoS
issue.

This patch adds a check for negative dentry.stream.valid_size to prevent
this vulnerability.

Co-developed-by: Seunghun Han <[e-mail]>
Signed-off-by: Seunghun Han <[e-mail]>
Co-developed-by: Jihoon Kwon <[e-mail]>
Signed-off-by: Jihoon Kwon <[e-mail]>
Signed-off-by: Jaehun Gou <[e-mail]>
Signed-off-by: Namjae Jeon <[e-mail]>

fs/exfat/namei.c

···
 
 	info->type = exfat_get_entry_type(ep);
 	info->attr = le16_to_cpu(ep->dentry.file.attr);
-	info->size = le64_to_cpu(ep2->dentry.stream.valid_size);
 	info->valid_size = le64_to_cpu(ep2->dentry.stream.valid_size);
 	info->size = le64_to_cpu(ep2->dentry.stream.size);
+
+	if (info->valid_size < 0) {
+		exfat_fs_error(sb, "data valid size is invalid(%lld)", info->valid_size);
+		return -EIO;
+	}
 
 	if (unlikely(EXFAT_B_TO_CLU_ROUND_UP(info->size, sbi) > sbi->used_clusters)) {
 		exfat_fs_error(sb, "data size is invalid(%lld)", info->size);