CVE-2021-47635 (GCVE-0-2021-47635)
Vulnerability from cvelistv5
Published
2025-02-26 01:54
Modified
2025-05-04 07:15
Severity ?
VLAI Severity ?
EPSS score ?
Summary
In the Linux kernel, the following vulnerability has been resolved:
ubifs: Fix to add refcount once page is set private
MM defined the rule [1] very clearly that once page was set with PG_private
flag, we should increment the refcount in that page, also main flows like
pageout(), migrate_page() will assume there is one additional page
reference count if page_has_private() returns true. Otherwise, we may
get a BUG in page migration:
page:0000000080d05b9d refcount:-1 mapcount:0 mapping:000000005f4d82a8
index:0xe2 pfn:0x14c12
aops:ubifs_file_address_operations [ubifs] ino:8f1 dentry name:"f30e"
flags: 0x1fffff80002405(locked|uptodate|owner_priv_1|private|node=0|
zone=1|lastcpupid=0x1fffff)
page dumped because: VM_BUG_ON_PAGE(page_count(page) != 0)
------------[ cut here ]------------
kernel BUG at include/linux/page_ref.h:184!
invalid opcode: 0000 [#1] SMP
CPU: 3 PID: 38 Comm: kcompactd0 Not tainted 5.15.0-rc5
RIP: 0010:migrate_page_move_mapping+0xac3/0xe70
Call Trace:
ubifs_migrate_page+0x22/0xc0 [ubifs]
move_to_new_page+0xb4/0x600
migrate_pages+0x1523/0x1cc0
compact_zone+0x8c5/0x14b0
kcompactd+0x2bc/0x560
kthread+0x18c/0x1e0
ret_from_fork+0x1f/0x30
Before the time, we should make clean a concept, what does refcount means
in page gotten from grab_cache_page_write_begin(). There are 2 situations:
Situation 1: refcount is 3, page is created by __page_cache_alloc.
TYPE_A - the write process is using this page
TYPE_B - page is assigned to one certain mapping by calling
__add_to_page_cache_locked()
TYPE_C - page is added into pagevec list corresponding current cpu by
calling lru_cache_add()
Situation 2: refcount is 2, page is gotten from the mapping's tree
TYPE_B - page has been assigned to one certain mapping
TYPE_A - the write process is using this page (by calling
page_cache_get_speculative())
Filesystem releases one refcount by calling put_page() in xxx_write_end(),
the released refcount corresponds to TYPE_A (write task is using it). If
there are any processes using a page, page migration process will skip the
page by judging whether expected_page_refs() equals to page refcount.
The BUG is caused by following process:
PA(cpu 0) kcompactd(cpu 1)
compact_zone
ubifs_write_begin
page_a = grab_cache_page_write_begin
add_to_page_cache_lru
lru_cache_add
pagevec_add // put page into cpu 0's pagevec
(refcnf = 3, for page creation process)
ubifs_write_end
SetPagePrivate(page_a) // doesn't increase page count !
unlock_page(page_a)
put_page(page_a) // refcnt = 2
[...]
PB(cpu 0)
filemap_read
filemap_get_pages
add_to_page_cache_lru
lru_cache_add
__pagevec_lru_add // traverse all pages in cpu 0's pagevec
__pagevec_lru_add_fn
SetPageLRU(page_a)
isolate_migratepages
isolate_migratepages_block
get_page_unless_zero(page_a)
// refcnt = 3
list_add(page_a, from_list)
migrate_pages(from_list)
__unmap_and_move
move_to_new_page
ubifs_migrate_page(page_a)
migrate_page_move_mapping
expected_page_refs get 3
(migration[1] + mapping[1] + private[1])
release_pages
put_page_testzero(page_a) // refcnt = 3
page_ref_freeze // refcnt = 0
page_ref_dec_and_test(0 - 1 = -1)
page_ref_unfreeze
VM_BUG_ON_PAGE(-1 != 0, page)
UBIFS doesn't increase the page refcount after setting private flag, which
leads to page migration task believes the page is not used by any other
processes, so the page is migrated. This causes concurrent accessing on
page refcount between put_page() called by other process(eg. read process
calls lru_cache_add) and page_ref_unfreeze() called by mi
---truncated---
References
Impacted products
| Vendor | Product | Version | |||||||
|---|---|---|---|---|---|---|---|---|---|
| ▼ | Linux | Linux |
Version: 1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d Version: 1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d Version: 1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d Version: 1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d Version: 1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d |
||||||
|
|||||||||
{
"containers": {
"cna": {
"affected": [
{
"defaultStatus": "unaffected",
"product": "Linux",
"programFiles": [
"fs/ubifs/file.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"lessThan": "c34ae24a2590fee96a3a7735ba2fa6cc52306221",
"status": "affected",
"version": "1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d",
"versionType": "git"
},
{
"lessThan": "4f75bab98565afd4f905059c56ec4caba88a7eec",
"status": "affected",
"version": "1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d",
"versionType": "git"
},
{
"lessThan": "5aaa2c0f0052b02c4a982993d4c5bb68fb7cbe22",
"status": "affected",
"version": "1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d",
"versionType": "git"
},
{
"lessThan": "fbeb2139eed65e929ce806c6468e6601ade01b1b",
"status": "affected",
"version": "1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d",
"versionType": "git"
},
{
"lessThan": "3b67db8a6ca83e6ff90b756d3da0c966f61cd37b",
"status": "affected",
"version": "1e51764a3c2ac05a23a22b2a95ddee4d9bffb16d",
"versionType": "git"
}
]
},
{
"defaultStatus": "affected",
"product": "Linux",
"programFiles": [
"fs/ubifs/file.c"
],
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"versions": [
{
"status": "affected",
"version": "2.6.27"
},
{
"lessThan": "2.6.27",
"status": "unaffected",
"version": "0",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.10.*",
"status": "unaffected",
"version": "5.10.110",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.15.*",
"status": "unaffected",
"version": "5.15.33",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.16.*",
"status": "unaffected",
"version": "5.16.19",
"versionType": "semver"
},
{
"lessThanOrEqual": "5.17.*",
"status": "unaffected",
"version": "5.17.2",
"versionType": "semver"
},
{
"lessThanOrEqual": "*",
"status": "unaffected",
"version": "5.18",
"versionType": "original_commit_for_fix"
}
]
}
],
"cpeApplicability": [
{
"nodes": [
{
"cpeMatch": [
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.10.110",
"versionStartIncluding": "2.6.27",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.15.33",
"versionStartIncluding": "2.6.27",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.16.19",
"versionStartIncluding": "2.6.27",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.17.2",
"versionStartIncluding": "2.6.27",
"vulnerable": true
},
{
"criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*",
"versionEndExcluding": "5.18",
"versionStartIncluding": "2.6.27",
"vulnerable": true
}
],
"negate": false,
"operator": "OR"
}
]
}
],
"descriptions": [
{
"lang": "en",
"value": "In the Linux kernel, the following vulnerability has been resolved:\n\nubifs: Fix to add refcount once page is set private\n\nMM defined the rule [1] very clearly that once page was set with PG_private\nflag, we should increment the refcount in that page, also main flows like\npageout(), migrate_page() will assume there is one additional page\nreference count if page_has_private() returns true. Otherwise, we may\nget a BUG in page migration:\n\n page:0000000080d05b9d refcount:-1 mapcount:0 mapping:000000005f4d82a8\n index:0xe2 pfn:0x14c12\n aops:ubifs_file_address_operations [ubifs] ino:8f1 dentry name:\"f30e\"\n flags: 0x1fffff80002405(locked|uptodate|owner_priv_1|private|node=0|\n zone=1|lastcpupid=0x1fffff)\n page dumped because: VM_BUG_ON_PAGE(page_count(page) != 0)\n ------------[ cut here ]------------\n kernel BUG at include/linux/page_ref.h:184!\n invalid opcode: 0000 [#1] SMP\n CPU: 3 PID: 38 Comm: kcompactd0 Not tainted 5.15.0-rc5\n RIP: 0010:migrate_page_move_mapping+0xac3/0xe70\n Call Trace:\n ubifs_migrate_page+0x22/0xc0 [ubifs]\n move_to_new_page+0xb4/0x600\n migrate_pages+0x1523/0x1cc0\n compact_zone+0x8c5/0x14b0\n kcompactd+0x2bc/0x560\n kthread+0x18c/0x1e0\n ret_from_fork+0x1f/0x30\n\nBefore the time, we should make clean a concept, what does refcount means\nin page gotten from grab_cache_page_write_begin(). There are 2 situations:\nSituation 1: refcount is 3, page is created by __page_cache_alloc.\n TYPE_A - the write process is using this page\n TYPE_B - page is assigned to one certain mapping by calling\n\t __add_to_page_cache_locked()\n TYPE_C - page is added into pagevec list corresponding current cpu by\n\t calling lru_cache_add()\nSituation 2: refcount is 2, page is gotten from the mapping\u0027s tree\n TYPE_B - page has been assigned to one certain mapping\n TYPE_A - the write process is using this page (by calling\n\t page_cache_get_speculative())\nFilesystem releases one refcount by calling put_page() in xxx_write_end(),\nthe released refcount corresponds to TYPE_A (write task is using it). If\nthere are any processes using a page, page migration process will skip the\npage by judging whether expected_page_refs() equals to page refcount.\n\nThe BUG is caused by following process:\n PA(cpu 0) kcompactd(cpu 1)\n\t\t\t\tcompact_zone\nubifs_write_begin\n page_a = grab_cache_page_write_begin\n add_to_page_cache_lru\n lru_cache_add\n pagevec_add // put page into cpu 0\u0027s pagevec\n (refcnf = 3, for page creation process)\nubifs_write_end\n SetPagePrivate(page_a) // doesn\u0027t increase page count !\n unlock_page(page_a)\n put_page(page_a) // refcnt = 2\n\t\t\t\t[...]\n\n PB(cpu 0)\nfilemap_read\n filemap_get_pages\n add_to_page_cache_lru\n lru_cache_add\n __pagevec_lru_add // traverse all pages in cpu 0\u0027s pagevec\n\t __pagevec_lru_add_fn\n\t SetPageLRU(page_a)\n\t\t\t\tisolate_migratepages\n isolate_migratepages_block\n\t\t\t\t get_page_unless_zero(page_a)\n\t\t\t\t // refcnt = 3\n list_add(page_a, from_list)\n\t\t\t\tmigrate_pages(from_list)\n\t\t\t\t __unmap_and_move\n\t\t\t\t move_to_new_page\n\t\t\t\t ubifs_migrate_page(page_a)\n\t\t\t\t migrate_page_move_mapping\n\t\t\t\t\t expected_page_refs get 3\n (migration[1] + mapping[1] + private[1])\n\t release_pages\n\t put_page_testzero(page_a) // refcnt = 3\n page_ref_freeze // refcnt = 0\n\t page_ref_dec_and_test(0 - 1 = -1)\n page_ref_unfreeze\n VM_BUG_ON_PAGE(-1 != 0, page)\n\nUBIFS doesn\u0027t increase the page refcount after setting private flag, which\nleads to page migration task believes the page is not used by any other\nprocesses, so the page is migrated. This causes concurrent accessing on\npage refcount between put_page() called by other process(eg. read process\ncalls lru_cache_add) and page_ref_unfreeze() called by mi\n---truncated---"
}
],
"providerMetadata": {
"dateUpdated": "2025-05-04T07:15:14.250Z",
"orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"shortName": "Linux"
},
"references": [
{
"url": "https://git.kernel.org/stable/c/c34ae24a2590fee96a3a7735ba2fa6cc52306221"
},
{
"url": "https://git.kernel.org/stable/c/4f75bab98565afd4f905059c56ec4caba88a7eec"
},
{
"url": "https://git.kernel.org/stable/c/5aaa2c0f0052b02c4a982993d4c5bb68fb7cbe22"
},
{
"url": "https://git.kernel.org/stable/c/fbeb2139eed65e929ce806c6468e6601ade01b1b"
},
{
"url": "https://git.kernel.org/stable/c/3b67db8a6ca83e6ff90b756d3da0c966f61cd37b"
}
],
"title": "ubifs: Fix to add refcount once page is set private",
"x_generator": {
"engine": "bippy-1.2.0"
}
}
},
"cveMetadata": {
"assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"assignerShortName": "Linux",
"cveId": "CVE-2021-47635",
"datePublished": "2025-02-26T01:54:09.701Z",
"dateReserved": "2025-02-26T01:48:21.518Z",
"dateUpdated": "2025-05-04T07:15:14.250Z",
"state": "PUBLISHED"
},
"dataType": "CVE_RECORD",
"dataVersion": "5.1"
}