# cveboard — Historical CVE brief

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Fix information unknown; no fixed version is derived here. Missing information does not prove that no fix exists.
Exploitation unknown; this catalogue does not assess exploitation.

## Source facts (JSON-quoted data)
CNA source text (English)
id: "CVE-2021-47275"
idYear: 2021
status: "PUBLISHED"
publishedAt: "2024-05-21T14:20:03.357Z"
updatedAt: "2026-05-11T13:51:21.584Z"
importedAt: "2026-10-24T01:56:36.661Z"
snapshot: {"id":"cve_2026-10-24_midnight_baseline","at":"2026-10-24T00:00:00.000Z","source":"https://github.com/CVEProject/cvelistV5/releases/tag/cve_2026-10-24_0100Z"}
url: "https://cveboard.org/cve/CVE-2021-47275"
json: "https://cveboard.org/cve/CVE-2021-47275.json"
sourceURL: "https://www.cve.org/CVERecord?id=CVE-2021-47275"
title: "bcache: avoid oversized read request in cache missing code path"
summary: "In the Linux kernel, the following vulnerability has been resolved:\n\nbcache: avoid oversized read request in cache missing code path\n\nIn the cache missing code path of cached device, if a proper location\nfrom the internal B+ tree is matched for a cache miss range, function\ncached_dev_cache_miss() will be called in cache_lookup_fn() in the\nfollowing code block,\n[code block 1]\n  526         unsigned int sectors = KEY_INODE(k) == s-\u003eiop.inode\n  527                 ? min_t(uint64_t, INT_MAX,\n  528                         KEY_START(k) - bio-\u003ebi_iter.bi_sector)\n  529                 : INT_MAX;\n  530         int ret = s-\u003ed-\u003ecache_miss(b, s, bio, sectors);\n\nHere s-\u003ed-\u003ecache_miss() is the call backfunction pointer initialized as\ncached_dev_cache_miss(), the last parameter 'sectors' is an important\nhint to calculate the size of read request to backing device of the\nmissing cache data.\n\nCurrent calculation in above code block may generate oversized value of\n'sectors', which consequently may trigger 2 different potential kernel\npanics by BUG() or BUG_ON() as listed below,\n\n1) BUG_ON() inside bch_btree_insert_key(),\n[code block 2]\n   886         BUG_ON(b-\u003eops-\u003eis_extents && !KEY_SIZE(k));\n2) BUG() inside biovec_slab(),\n[code block 3]\n   51         default:\n   52                 BUG();\n   53                 return NULL;\n\nAll the above panics are original from cached_dev_cache_miss() by the\noversized parameter 'sectors'.\n\nInside cached_dev_cache_miss(), parameter 'sectors' is used to calculate\nthe size of data read from backing device for the cache missing. This\nsize is stored in s-\u003einsert_bio_sectors by the following lines of code,\n[code block 4]\n  909    s-\u003einsert_bio_sectors = min(sectors, bio_sectors(bio) + reada);\n\nThen the actual key inserting to the internal B+ tree is generated and\nstored in s-\u003eiop.replace_key by the following lines of code,\n[code block 5]\n  911   s-\u003eiop.replace_key = KEY(s-\u003eiop.inode,\n  912                    bio-\u003ebi_iter.bi_sector + s-\u003einsert_bio_sectors,\n  913                    s-\u003einsert_bio_sectors);\nThe oversized parameter 'sectors' may trigger panic 1) by BUG_ON() from\nthe above code block.\n\nAnd the bio sending to backing device for the missing data is allocated\nwith hint from s-\u003einsert_bio_sectors by the following lines of code,\n[code block 6]\n  926    cache_bio = bio_alloc_bioset(GFP_NOWAIT,\n  927                 DIV_ROUND_UP(s-\u003einsert_bio_sectors, PAGE_SECTORS),\n  928                 &dc-\u003edisk.bio_split);\nThe oversized parameter 'sectors' may trigger panic 2) by BUG() from the\nagove code block.\n\nNow let me explain how the panics happen with the oversized 'sectors'.\nIn code block 5, replace_key is generated by macro KEY(). From the\ndefinition of macro KEY(),\n[code block 7]\n  71 #define KEY(inode, offset, size)                                  \\\n  72 ((struct bkey) {                                                  \\\n  73      .high = (1ULL \u003c\u003c 63) | ((__u64) (size) \u003c\u003c 20) | (inode),     \\\n  74      .low = (offset)                                              \\\n  75 })\n\nHere 'size' is 16bits width embedded in 64bits member 'high' of struct\nbkey. But in code block 1, if \"KEY_START(k) - bio-\u003ebi_iter.bi_sector\" is\nvery probably to be larger than (1\u003c\u003c16) - 1, which makes the bkey size\ncalculation in code block 5 is overflowed. In one bug report the value\nof parameter 'sectors' is 131072 (= 1 \u003c\u003c 17), the overflowed 'sectors'\nresults the overflowed s-\u003einsert_bio_sectors in code block 4, then makes\nsize field of s-\u003eiop.replace_key to be 0 in code block 5. Then the 0-\nsized s-\u003eiop.replace_key is inserted into the internal B+ tree as cache\nmissing check key (a special key to detect and avoid a racing between\nnormal write request and cache missing read request) as,\n[code block 8]\n  915   ret = bch_btree_insert_check_key(b, &s-\u003eop, &s-\u003eiop.replace_key);\n\nThen the 0-sized s-\u003eiop.replace_key as 3rd parameter triggers the bkey\nsize check BUG_ON() in code block 2, and causes the kernel panic 1).\n\nAnother ke\n---truncated---"
affected: [{"versions":[{"version":"cafe563591446cf80bfbc2fe3bc72a2e36cf1060","status":"affected","versionType":"git","lessThan":"555002a840ab88468e252b0eedf0b05e2ce7099c"},{"version":"cafe563591446cf80bfbc2fe3bc72a2e36cf1060","status":"affected","versionType":"git","lessThan":"41fe8d088e96472f63164e213de44ec77be69478"}],"vendor":"Linux","product":"Linux","defaultStatus":"unaffected"},{"versions":[{"version":"3.10","status":"affected"},{"version":"0","status":"unaffected","versionType":"semver","lessThan":"3.10"},{"version":"5.12.11","status":"unaffected","versionType":"semver","lessThanOrEqual":"5.12.*"},{"version":"5.13","status":"unaffected","versionType":"original_commit_for_fix","lessThanOrEqual":"*"}],"vendor":"Linux","product":"Linux","defaultStatus":"affected"}]

## Source licences and attribution
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GitHub Advisory Database: https://github.com/advisories (CC BY 4.0)
Have I Been Pwned: https://haveibeenpwned.com/ (CC BY 4.0)
CVE Program (CVE records): https://www.cve.org/Legal/TermsOfUse (CVE Terms of Use)
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