ksmbd: fix integer overflow in set_file_allocation_info()

set_file_allocation_info() converts the client-supplied FILE_ALLOCATION_INFORMATION::AllocationSize into a 512-byte block count with:

	alloc_blks = (le64_to_cpu(file_alloc_info->AllocationSize) + 511) >> 9;

AllocationSize is a fully client-controlled __le64 field; the only validation performed by the caller (smb2_set_info_file(), case FILE_ALLOCATION_INFORMATION) is that the fixed buffer is at least sizeof(struct smb2_file_alloc_info) == 8 bytes. The value itself is never range-checked before this arithmetic.

When AllocationSize is close to U64_MAX (e.g. 0xffffffffffffffff), "AllocationSize + 511" wraps around mod 2^64 to a small number (0xffffffffffffffff + 511 = 510), so alloc_blks becomes 0. Since any existing regular file has stat.blocks > 0, the function then takes the "shrink" branch and calls:

	ksmbd_vfs_truncate(work, fp, alloc_blks * 512);   /* == 0 */

silently truncating the file to size 0, even though the client asked to grow the allocation to (what looks like) the maximum possible size. The trailing "if (size < alloc_blks * 512) i_size_write(inode, size);" restore is guarded by a comparison that is never true once alloc_blks == 0, so the truncation is not undone. This lets an authenticated SMB client that already holds an open handle with FILE_WRITE_DATA on a file silently truncate that same file to size 0 via a single crafted SET_INFO(FILE_ALLOCATION_INFORMATION) request advertising a near-U64_MAX AllocationSize, even though the request asks to grow the file's allocation rather than shrink it. This is a functional/data-loss bug, not a privilege-boundary
violation: the same client could already truncate the file via FILE_END_OF_FILE_INFORMATION or a plain write.

Fix it by validating AllocationSize against MAX_LFS_FILESIZE, the same upper bound the VFS itself uses to reject unrepresentable file sizes, before doing the "+511" rounding, and rejecting oversized values with -EINVAL. Bounding AllocationSize to MAX_LFS_FILESIZE - 511 guarantees the "+511" addition cannot wrap, and that the subsequent "alloc_blks * 512" values passed to vfs_fallocate() and ksmbd_vfs_truncate() stay within a representable loff_t as well.

No legitimate SMB client asks for an allocation size anywhere near 2^64 bytes, so this only rejects a value that was previously silently misinterpreted as zero.

Runtime-verified on a v6.19 KASAN test stand: sending SET_INFO (FILE_ALLOCATION_INFORMATION) with AllocationSize = 0xffffffffffffffff against ksmbd now returns -EINVAL and leaves the target file's size unchanged, where the unpatched kernel truncated it from 4096 to 0 bytes.

---
 fs/smb/server/smb2pdu.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

index 097f51fc7ed6..324916533669 100644
@@ -6690,6 +6690,7 @@ static int set_file_allocation_info(struct ksmbd_work *work,
 	 */
 
 	loff_t alloc_blks;
+	u64 alloc_size;
 	struct inode *inode;
 	struct kstat stat;
 	int rc;

@@ -6705,7 +6706,19 @@ static int set_file_allocation_info(struct ksmbd_work *work,
 	if (rc)
 		return rc;
 
-	alloc_blks = (le64_to_cpu(file_alloc_info->AllocationSize) + 511) >> 9;
+	/*
+	 * AllocationSize is fully client-controlled (the caller only
+	 * validates the fixed 8-byte buffer length). Reject values that
+	 * would overflow the "round up to 512-byte blocks" conversion
+	 * below instead of silently wrapping it to a tiny block count,
+	 * which would truncate the file to a size the client never
+	 * asked for.
+	 */
+	alloc_size = le64_to_cpu(file_alloc_info->AllocationSize);
+	if (alloc_size > MAX_LFS_FILESIZE - 511)
+		return -EINVAL;
+
+	alloc_blks = (alloc_size + 511) >> 9;
 	inode = file_inode(fp->filp);
 
 	if (alloc_blks > stat.blocks) {

2.50.1 (Apple Git-155)

Applied it to #ksmbd-for-next-next.
Thanks!