Bug 2562763 (CVE-2026-99109)

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

smb: client: reject SetEA requests that do not fit the request buffer

CIFSSMBSetEA() copies the caller's extended attribute value into the
SMB request buffer without checking that it fits.  The requirement is
stated in the source but was never implemented:

	/*BB add length check to see if it would fit in
	     negotiated SMB buffer size BB */
	/* if (ea_value_len > buffer_size - 512 (enough for header)) */
	if (ea_value_len)
		memcpy(parm_data->list.name + name_len + 1,
		       ea_value, ea_value_len);

The only bound applied on the way in is in cifs_xattr_set():

	#define MAX_EA_VALUE_SIZE CIFSMaxBufSize
	...
	if (size > MAX_EA_VALUE_SIZE)

CIFSMaxBufSize is the full payload capacity of the buffer, so a value
of exactly that size leaves no room for the SMB header, the TRANS2
parameter block, the fealist header and the EA name that are written
ahead of it in the same object.

SendReceive() already enforces the correct limit on this very length:

	if (in_len > CIFSMaxBufSize + MAX_CIFS_HDR_SIZE)

but it is called after the copy has taken place.  An unprivileged
setxattr(2) on an SMB1 mount with a 250-byte name and a 16384-byte
value writes 16384 bytes starting 345 bytes into a 16588-byte
cifs_request object, ending 141 bytes past it:

  BUG: KASAN: slab-out-of-bounds in CIFSSMBSetEA+0xabc/0xde0
  Write of size 16384 at addr ffff888003aa0159 by task init/68
   __asan_memcpy+0x3c/0x60
   CIFSSMBSetEA+0xabc/0xde0
   cifs_xattr_set+0xd3a/0xff0
   __vfs_setxattr+0x13e/0x1a0
  The buggy address is located 345 bytes inside of
   allocated 16588-byte region

Apply SendReceive()'s limit to the assembled request before the copy
rather than after it, and widen the byte counters so the sum cannot
wrap before it is tested.

byte_count is also tested against U16_MAX, because it is stored in the
16-bit pSMB->ByteCount.  That becomes reachable when CIFSMaxBufSize is
raised at module load, where it may be set as high as 1024*127: with a
5-byte EA name and a 65521-byte value, count is exactly U16_MAX while
byte_count is 65556, and cpu_to_le16() would truncate it to 20 and
transmit a frame whose ByteCount does not match its length.  Testing
byte_count covers count as well, since byte_count is the larger of the
two and count's only 16-bit consumer is written after this point.

check_add_overflow() is evaluated first so that total_len is assigned
before it is reported.