Dear sir, We are the ADLab, Venustech info Ltd CHINA. We have found a vulnerability within linux kernel.Please check out the detail about this vulnerability at the end of this Email. The CVE said that we could get the CVE number from you if the vulnerabilities are confirmed.Please send me a confirm email with the CVE number if it is possible.Thank you. We are looking forward to hearing from you as soon as possible. If you need any more information, please do not hesitate to contact us. Thank you very much. Best regards, ADLab Details: ********************************************************* Linux kernel Filesystem Mount Dead Loop [Security Advisory] Advisory: [AD_LAB-06011] Linux kernel Filesystem Mount Dead Loop Class: Design Error DATE:5/9/2006 CVEID: Vulnerable: Linux-2.6.* Vendor: http://www.kernel.org/ I.DESCRIPTION: ------------- Linux kernel is an open source operating system. The Linux kernel is prone to a denial-of-service vulnerability. This issue is due to a design error in the '__getblk()' function. II.DETAILS: ---------- There is a vulnerability in function __getblk(). When mount a file system image with malformed block value, Linux kernel will fall in a dead loop. It will lead to a kernel hang and denying further service. Function __getblk() is used to seek a corresponding buffer_head of a block in a specific block device. When processing a block with a block number more than 4G and not to be mapped to buffer pages (__find_get_block will return NULL), __getblk_slow will always run and never return. 1478 struct buffer_head * 1479 __getblk(struct block_device *bdev, sector_t block, int size) 1480 { 1481 struct buffer_head *bh = __find_get_block(bdev, block, size); 1482 1483 might_sleep(); 1484 if (bh == NULL) 1485 bh = __getblk_slow(bdev, block, size); 1486 return bh; 1487 } The endless for loop only has a exit point (at line 1213). It terminate only when __find_get_block return a non-NULL value. If the block is not mapped to buffer pages, the first __find_get_block calling will return a NULL bh and the function grow_buffers() will be called subsequently. 1194 __getblk_slow(struct block_device *bdev, sector_t block, int size) 1195 { 1196 /* Size must be multiple of hard sectorsize */ 1197 if (unlikely(size & (bdev_hardsect_size(bdev)-1) || 1198 (size < 512 || size > PAGE_SIZE))) { 1199 printk(KERN_ERR "getblk(): invalid block size %d requested\n", 1200 size); 1201 printk(KERN_ERR "hardsect size: %d\n", 1202 bdev_hardsect_size(bdev)); 1203 1204 dump_stack(); 1205 return NULL; 1206 } 1207 1208 for (;;) { 1209 struct buffer_head * bh; 1210 1211 bh = __find_get_block(bdev, block, size); 1212 if (bh) 1213 return bh; 1214 1215 if (!grow_buffers(bdev, block, size)) 1216 free_more_memory(); 1217 } 1218 } The function grow_buffers() is responsible for construct the relationships among page, buffer_head and block. On the 32-bit platform, the length of block and index are 64-bit and 32-bit respectively. After the operations at line 1201 and 1202, the high 32 bits of block will lost. Consequently, when the block number is beyond 4G, new block number would be differently with the original. 1189 static inline int 1190 grow_buffers(struct block_device *bdev, sector_t block, int size) 1191 { 1192 struct page *page; 1193 pgoff_t index; 1194 int sizebits; 1195 1196 sizebits = -1; 1197 do { 1198 sizebits++; 1199 } while ((size << sizebits) < PAGE_SIZE); 1200 1201 index = block >> sizebits; 1202 block = index << sizebits; 1203 1204 /* Create a page with the proper size buffers.. */ 1205 page = grow_dev_page(bdev, block, index, size); 1206 if (!page) 1207 return 0; 1208 unlock_page(page); 1209 page_cache_release(page); 1210 return 1; 1211 } Follow the call sequence (grow_dev_page ==> init_page_buffers ==> init_page_buffers). In init_page_buffers(), a new buffer_head will be initialized, it's block number (bh->b_blocknr) corresponding to mapped block is assigned with the new block number. static void init_page_buffers(struct page *page, struct block_device *bdev, sector_t block, int size) { ... ... do { if (!buffer_mapped(bh)) { init_buffer(bh, NULL, NULL); bh->b_bdev = bdev; bh->b_blocknr = block; if (uptodate) set_buffer_uptodate(bh); set_buffer_mapped(bh); } block++; bh = bh->b_this_page; } while (bh != head); } However, __find_get_block seeks buffer head base on the original block number in __getblk_slow (). Because of wrong block number argument, __find_get_block will always return NULL. As results, system will fall in a dead loop and consume resource endlessly. 1194 __getblk_slow(struct block_device *bdev, sector_t block, int size) 1195 { ... ... 1207 1208 for (;;) { 1209 struct buffer_head * bh; 1210 1211 bh = __find_get_block(bdev, block, size); 1212 if (bh) 1213 return bh; 1214 1215 if (!grow_buffers(bdev, block, size)) 1216 free_more_memory(); 1217 } 1218 } example： The vulnerability can be triggered by mount a malformed Reiser filesystem image, the arguments of __getblk() are: __getblk(0xcd0ed0c0, 0xffffffffa10020d9, 0x1000) call stack: sys_mount -> do_mount -> do_kern_mount -> get_super_block -> get_sb_bdev -> reiserfs_fill_super -> reiserfs_read_locked_inode -> search_by_key -> __getblk -> __find_get_block III.CREDIT: ---------- Venustech AD-LAB discovery this vuln. Thank to all Venustech AD-Lab guys. V.DISCLAIMS: ----------- The information in this bulletin is provided "AS IS" without warranty of any kind. In no event shall we be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages. Copyright 1996-2006 VENUSTECH. All Rights Reserved. Terms of use. VENUSTECH Security Lab VENUSTECH INFORMATION TECHNOLOGY CO.,LTD(http://www.venustech.com.cn) Security Trusted {Solution} Provider Service ********************************************************* - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
