Check read_mapping_page return value in hfsplus

hi,

while testing more corrupted images with hfsplus, i came across one which triggered the following bug:

[15840.675016] BUG: unable to handle kernel paging request at fffffffb
[15840.675016] IP: [<c0116a4f>] kmap+0x15/0x56
[15840.675016] *pde = 00008067 *pte = 00000000
[15840.675016] Oops: 0000 [#1] PREEMPT DEBUG_PAGEALLOC
[15840.675016] Modules linked in:
[15840.675016]
[15840.675016] Pid: 11575, comm: ln Not tainted (2.6.27-rc4-00123-gd3ee1b4-dirty #29)
[15840.675016] EIP: 0060:[<c0116a4f>] EFLAGS: 00010202 CPU: 0
[15840.675016] EIP is at kmap+0x15/0x56
[15840.675016] EAX: 00000246 EBX: fffffffb ECX: 00000000 EDX: cab919c0
[15840.675016] ESI: 000007dd EDI: cab0bcf4 EBP: cab0bc98 ESP: cab0bc94
[15840.675016]  DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 0068
[15840.675016] Process ln (pid: 11575, ti=cab0b000 task=cab919c0 task.ti=cab0b000)
[15840.675016] Stack: 00000000 cab0bcdc c0231cfb 00000000 cab0bce0 00000800 ca9290c0 fffffffb
[15840.675016]        cab145d0 cab919c0 cab15998 22222222 22222222 22222222 00000001 cab15960
[15840.675016]        000007dd cab0bcf4 cab0bd04 c022cb3a cab0bcf4 cab15a6c ca9290c0 00000000
[15840.675016] Call Trace:
[15840.675016]  [<c0231cfb>] ? hfsplus_block_allocate+0x6f/0x2d3
[15840.675016]  [<c022cb3a>] ? hfsplus_file_extend+0xc4/0x1db
[15840.675016]  [<c022ce41>] ? hfsplus_get_block+0x8c/0x19d
[15840.675016]  [<c06adde4>] ? sub_preempt_count+0x9d/0xab
[15840.675016]  [<c019ece6>] ? __block_prepare_write+0x147/0x311
[15840.675016]  [<c0161934>] ? __grab_cache_page+0x52/0x73
[15840.675016]  [<c019ef4f>] ? block_write_begin+0x79/0xd5
[15840.675016]  [<c022cdb5>] ? hfsplus_get_block+0x0/0x19d
[15840.675016]  [<c019f22a>] ? cont_write_begin+0x27f/0x2af
[15840.675016]  [<c022cdb5>] ? hfsplus_get_block+0x0/0x19d
[15840.675016]  [<c0139ebe>] ? tick_program_event+0x28/0x4c
[15840.675016]  [<c013bd35>] ? trace_hardirqs_off+0xb/0xd
[15840.675016]  [<c022b723>] ? hfsplus_write_begin+0x2d/0x32
[15840.675016]  [<c022cdb5>] ? hfsplus_get_block+0x0/0x19d
[15840.675016]  [<c0161988>] ? pagecache_write_begin+0x33/0x107
[15840.675016]  [<c01879e5>] ? __page_symlink+0x3c/0xae
[15840.675016]  [<c019ad34>] ? __mark_inode_dirty+0x12f/0x137
[15840.675016]  [<c0187a70>] ? page_symlink+0x19/0x1e
[15840.675016]  [<c022e6eb>] ? hfsplus_symlink+0x41/0xa6
[15840.675016]  [<c01886a9>] ? vfs_symlink+0x99/0x101
[15840.675016]  [<c018a2f6>] ? sys_symlinkat+0x6b/0xad
[15840.675016]  [<c018a348>] ? sys_symlink+0x10/0x12
[15840.675016]  [<c01038bd>] ? sysenter_do_call+0x12/0x31
[15840.675016]  =======================
[15840.675016] Code: 00 00 75 10 83 3d 88 2f ec c0 02 75 07 89 d0 e8 12 56 05 00 5d c3 55 ba 06 00 00 00 89 e5 53 89 c3 b8 3d eb 7e c0 e8 16 74 00 00 <8b> 03 c1 e8 1e 69 c0 d8 02 00 00 05 b8 69 8e c0 2b 80 c4 02 00
[15840.675016] EIP: [<c0116a4f>] kmap+0x15/0x56 SS:ESP 0068:cab0bc94
[15840.675016] ---[ end trace 4fea40dad6b70e5f ]---

This happens because the return value of read_mapping_page() is passed on to kmap unchecked. The bug is triggered after the first read_mapping_page() in hfsplus_block_allocate(), this patch fixes all three usages in this functions but leaves the ones further down in the file unchanged.

@@ -32,6 +32,10 @@ int hfsplus_block_allocate(struct super_
 	mutex_lock(&HFSPLUS_SB(sb).alloc_file->i_mutex);
 	mapping = HFSPLUS_SB(sb).alloc_file->i_mapping;
 	page = read_mapping_page(mapping, offset / PAGE_CACHE_BITS, NULL);
+	if (IS_ERR(page)) {
+		start = size;
+		goto out;
+	}
 	pptr = kmap(page);
 	curr = pptr + (offset & (PAGE_CACHE_BITS - 1)) / 32;
 	i = offset % 32;

@@ -73,6 +77,10 @@ int hfsplus_block_allocate(struct super_
 			break;
 		page = read_mapping_page(mapping, offset / PAGE_CACHE_BITS,
 					 NULL);
+		if (IS_ERR(page)) {
+			start = size;
+			goto out;
+		}
 		curr = pptr = kmap(page);
 		if ((size ^ offset) / PAGE_CACHE_BITS)
 			end = pptr + PAGE_CACHE_BITS / 32;

@@ -120,6 +128,10 @@ found:
 		offset += PAGE_CACHE_BITS;
 		page = read_mapping_page(mapping, offset / PAGE_CACHE_BITS,
 					 NULL);
+		if (IS_ERR(page)) {
+			start = size;
+			goto out;
+		}
 		pptr = kmap(page);
 		curr = pptr;
 		end = pptr + PAGE_CACHE_BITS / 32;