[pve-devel] [PATCH cluster 2/2] clusterlog: fix segfault / wrong iteration bounds

Fabian Grünbichler f.gruenbichler at proxmox.com
Tue Dec 14 11:19:12 CET 2021


the clusterlog struct is a basic ring buffer:

struct clog_base {
    uint32_t size; // total size of this clog_base
    uint32_t cpos; // index into data, starts counting at start of clog_base, initially 0
    char data[];
};

an entry consists of indices of the next and previous entries and
various fields (fixed-length ones omitted here):

typedef struct {
	uint32_t prev; // index of previous entry, or 0 if none exists
	uint32_t next; // index of next entry
	[..] // fixed-length fields
	uint8_t node_len;
	uint8_t ident_len;
	uint8_t tag_len;
	uint32_t msg_len;
	char data[]; // node+ident+tag+msg - variable-length fields
} clog_entry_t;

the next and prev indices are calculated when allocating a new entry,
and the position of the current entry 'cpos' is updated accordingly
(clog_alloc_entry):
- size of the entry is padded with up to 7 bytes
- first entry goes to index 8
- second and subsequent entries go to the current entry's 'next' index
- if the current entry's 'next' index is out of bonds, the first entry
  is overwritten => wrap-around
- the 'prev' index of the new entry is set to cpos
- cpos is set to the index of the new entry
- the 'next' index of the new entry is set to its index+padded size

when iterating over the entries, the following bounds are used to follow
the 'prev' links starting at the current entry:

	while (cpos && (cpos <= clog->cpos || cpos > (clog->cpos + CLOG_MAX_ENTRY_SIZE))) {

while this handles a not-yet-wrapped around ring buffer (cpos would be 0
when reaching the first entry), and tries to handle wrap-arounds by
terminating when reaching a 'red-zone' of 'CLOG_MAX_ENTRY_SIZE' starting
at the current entry (this covers the current entry which was already
visited as first entry during the iteration, and the next entry after it
which might have been overwritten) - but it's possible that entries line
up so that the wrap-around 'prev' index of the first entry points to a
location *before* the current entry.

for example, looking at clog_base with S being the size field, C being
the cpos field, followed by the actual data. N/P are the next/prev
indices of the entry at C, Q denotes the 'prev' index of the first entry
in the data array, and 'R' the red zone used for the loop check in case
of wrap-around.

first, fill up the buffer with six large entries:

Q                               P      C      N
|                               |      |      |
|                               |      |      |
v                               v      v      v
+-+-+------+------+------+------+------+------+-+
| | |      |      |      |      |      |      |x|
| | |   1  |   2  |   3  |   4  |   5  |   6  |x|
| | |      |      |      |      |      |      |x|
+-+-+------+------+------+------+------+------+-+
 S C                                    RRRRRRRRRRR

iterating from C backwards ends up at Q being 0, terminating the loop
without a wrap-around after having visit 6->1

now the next (in this example, smaller) entry that gets allocated/insert
needs to wrap around, because the empty space at the end (denoted by
XXX) is too small:

    C      N                          QP
    |      |                          ||
    |      |                          ||
    v      v                          vv
+-+-+------+------+------+------+------+------+-+
| | |      |      |      |      |      |      |x|
| | |   7  |   2  |   3  |   4  |   5  |   6  |x|
| | |      |      |      |      |      |      |x|
+-+-+------+------+------+------+------+------+-+
 S C RRRRRRRRRRR

iterating backwards from C terminates the loop when reaching the red
zone, with the (second) entry no longer being considered since it partly
overlaps it. only 7->3 are visited.

adding more entries we end up with the following layout:

                                   P  QC   N
                                   |  ||   |
                                   |  ||   |
                                   v  vv   v
+-+-+------+---+---+---+---+---+---+---+---+--+-+
| | |      |   |   |   |   |   |   |   |   |##|x|
| | |   7  | 8 | 9 |10 |11 |12 |13 |14 |15 |#6|x|
| | |      |   |   |   |   |   |   |   |   |##|x|
+-+-+------+---+---+---+---+---+---+---+---+--+-+
 S C                                    RRRRRRRRRRR

with # denoting space previously occupied the last large entry (#6)
which is still unmodified (the rest of that entry's data has been
overwritten by entries #14 and #15).

iterating from C (to the left/P) the loop ends up at entry #7, follows
the link to Q (which satisfies the loop bounds as Q < C), and the data
starting at (invalid index) Q gets interpreted as an entry. it is
possible (though even more unlikely than the partial overwrite case)
that Q and C line up perfectly, which would cause the loop to become an
infinite loop. the loop *should* terminate after having visited 15-7,
without wrapping around.

note that the actual sizes of the entries are not relevant, the
requirements are:
- entry before last wrap-around must be big enough that entry of current
  index can overtake it without another wrap-around
- method that does iteration must be called before next wrap-around

the fix is obviously trivial once the issue became apparent - when
wrapping around during iteration, additionally check that we are not
jumping across the red zone into already invalidated parts of data.

clusterlog_merge is technically not affected since it aborts before a
wrap-around anyway, but it doesn't hurt to have the checks consistently
in case this ever changes.

thanks to @kev1904 on our community forums for reporting and providing the data
to nail the cause down fast!

Signed-off-by: Fabian Grünbichler <f.gruenbichler at proxmox.com>
---
 data/src/logger.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/data/src/logger.c b/data/src/logger.c
index 4cf9cce..619e1f6 100644
--- a/data/src/logger.c
+++ b/data/src/logger.c
@@ -136,6 +136,11 @@ clog_dump(clog_base_t *clog)
 	while (cpos && (cpos <= clog->cpos || cpos > (clog->cpos + CLOG_MAX_ENTRY_SIZE))) {
 		clog_entry_t *cur = (clog_entry_t *)((char *)clog + cpos);
 		clog_dump_entry(cur, cpos);
+
+		// wrap-around has to land after initial position
+		if (cpos < cur->prev && cur->prev <= clog->cpos) {
+			break;
+		}
 		cpos = cur->prev;
 	}
 }
@@ -165,6 +170,11 @@ clog_dump_json(
 	guint count = 0;
 	while (cpos && (cpos <= clog->cpos || cpos > (clog->cpos + CLOG_MAX_ENTRY_SIZE))) {
 		clog_entry_t *cur = (clog_entry_t *)((char *)clog + cpos);
+
+		// wrap-around has to land after initial position
+		if (cpos < cur->prev && cur->prev <= clog->cpos) {
+			break;
+		}
 		cpos = cur->prev;
 
 		if (count >= max_entries)
@@ -358,6 +368,11 @@ clog_sort(clog_base_t *clog)
 
 		g_tree_insert(tree, cur, cur);
 
+		// wrap-around has to land after initial position
+		if (cpos < cur->prev && cur->prev <= clog->cpos) {
+			break;
+		}
+
 		cpos = cur->prev;
 	}
 
@@ -509,10 +524,12 @@ clusterlog_merge(
 				break;
 		}
 
-		if (!cur->prev) {
+		// no previous entry or wrap-around into already overwritten entry
+		if (!cur->prev || (cpos[found] < cur->prev && cur->prev <= clog[found]->cpos)) {
 			cpos[found] = 0;
 		} else {
 			cpos[found] = cur->prev;
+			// wrap-around into current entry
 			if (!(cpos[found] <= clog[found]->cpos || 
 			      cpos[found] > (clog[found]->cpos + CLOG_MAX_ENTRY_SIZE))) {
 				cpos[found] = 0;
-- 
2.30.2





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