smb: client: fix fiemap wild read on empty truncated response

Overview

In the Linux kernel, the following vulnerability has been resolved:

smb: client: fix fiemap wild read on empty truncated response

smb3_fiemap() treats -E2BIG from SMB2_ioctl() as a truncated range blob and loops for the next chunk. If the server reports STATUS_BUFFER_OVERFLOW with OutputCount == 0, SMB2_ioctl() returns -E2BIG with out_data == NULL and out_data_len == 0, and the existing length checks accept the empty result. The loop then evaluates

next = out_data[num - 1].file_offset + ...

with num == 0, reading 16 bytes before NULL.

Reject a truncated range blob that contains no range, so the resume offset is never computed from an empty response. Also require the last range in a truncated blob to advance the resume offset without overflowing, and to stay within the requested range; a malicious or broken server could otherwise make no progress or underflow len and loop forever. Free the previous response buffer before retrying.

Reproduced on 6.6.87.2 with a malicious SMB2 server that answers FSCTL_QUERY_ALLOCATED_RANGES with STATUS_BUFFER_OVERFLOW and OutputCount == 0; an FS_IOC_FIEMAP ioctl on the mount oopses in cifs_fiemap() with CR2 = fffffffffffffff8. The code path is unchanged on mainline.

Affected

• Linux >= 2f3ebaba13cebd8badfb9aed31c0cf3cc82eb4f4 < 4a426ca5be7e3dedbe76575b967db04fbf01136f
• Linux >= 2f3ebaba13cebd8badfb9aed31c0cf3cc82eb4f4 < 31f6dbb29fe17fa9595c7e8b19fd92915b03a4ca
• Linux >= 2f3ebaba13cebd8badfb9aed31c0cf3cc82eb4f4 < f5baa17a0f316c235d61623ac8e8fb32c2b3c0cc
• Linux 5.2

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.