[Bug network/19865] New: Memory leak in _nss_dns_getcanonname_r

["fweimer at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19865

            Bug ID: 19865
           Summary: Memory leak in _nss_dns_getcanonname_r
           Product: glibc
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---
             Flags: security-

If the A lookup fails, but returns a response large than 20 bytes (so that
malloc happens in __libc_res_nquery), __libc_res_nquery is called again, with
the old, minimal buffer size, and allocates a new fresh buffer, overwriting the
pointer to the first allocated buffer, so that the allocated memory is now
lost.

This does not appear to be a recent regression.

_nss_dns_getcanonname_r seems to be dead code because nss_dns always provides a
canonical name directly, so this needs a rather unusual setup before this turns
into a security issue (denial of service due to memory exhaustion), which is
why I'm flagging this security-.

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