[xen stable-4.15] tools/xenstored: domain_entry_fix(): Handle conflicting transaction

[patchbot@xxxxxxx]

commit 0a70ce96deebbe3074708b618bab11cd802491c8
Author:     Julien Grall <jgrall@xxxxxxxxxx>
AuthorDate: Fri Sep 22 11:32:16 2023 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Sep 27 16:30:18 2023 +0100

    tools/xenstored: domain_entry_fix(): Handle conflicting transaction
    
    The function domain_entry_fix() will be initially called to check if the
    quota is correct before attempt to commit any nodes. So it would be
    possible that accounting is temporarily negative. This is the case
    in the following sequence:
    
      1) Create 50 nodes
      2) Start two transactions
      3) Delete all the nodes in each transaction
      4) Commit the two transactions
    
    Because the first transaction will have succeed and updated the
    accounting, there is no guarantee that 'd->nbentry + num' will still
    be above 0. So the assert() would be triggered.
    The assert() was introduced in dbef1f748289 ("tools/xenstore: simplify
    and fix per domain node accounting") with the assumption that the
    value can't be negative. As this is not true revert to the original
    check but restricted to the path where we don't update. Take the
    opportunity to explain the rationale behind the check.
    
    This CVE-2023-34323 / XSA-440.
    
    Fixes: dbef1f748289 ("tools/xenstore: simplify and fix per domain node 
accounting")
    Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
    (cherry picked from commit c4e05c97f57d236040d1da5c1fbf6e3699dc86ea)
---
 tools/xenstore/xenstored_domain.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/tools/xenstore/xenstored_domain.c 
b/tools/xenstore/xenstored_domain.c
index 8cc36ee44c..697a7cfe64 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/tools/xenstore/xenstored_domain.c
@@ -1068,10 +1068,20 @@ int domain_entry_fix(unsigned int domid, int num, bool 
update)
        }
 
        cnt = d->nbentry + num;
-       assert(cnt >= 0);
 
-       if (update)
+       if (update) {
+               assert(cnt >= 0);
                d->nbentry = cnt;
+       } else if (cnt < 0) {
+               /*
+                * In a transaction when a node is being added/removed AND
+                * the same node has been added/removed outside the
+                * transaction in parallel, the result value may be negative.
+                * This is no problem, as the transaction will fail due to
+                * the resulting conflict. So override 'cnt'.
+                */
+               cnt = 0;
+       }
 
        return domid_is_unprivileged(domid) ? cnt : 0;
 }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.15