Privilege escalation can deadlock

[mjstapp]

Privilege elevation is process-wide; if a multithreaded process has code paths that allow multiple threads to race to call the elevation api, the process can deadlock.

Here's an example backtrace, from zebra:

(gdb) bt
#0 futex_wait (private=0, expected=1, futex_word=0xffffa9026230) at ../sysdeps/unix/sysv/linux/futex-internal.h:61
#1 futex_wait_simple (private=0, expected=1, futex_word=0xffffa9026230) at ../sysdeps/nptl/futex-internal.h:135
#2 __nptl_setxid (cmdp=0xffffa9026210) at allocatestack.c:1180
#3 0x0000ffffa9453818 in __GI_seteuid (uid=) at ../sysdeps/unix/sysv/linux/seteuid.c:34
#4 __GI_seteuid (uid=) at ../sysdeps/unix/sysv/linux/seteuid.c:24
#5 0x0000ffffa97a025c in zprivs_change_uid (op=ZPRIVS_LOWER) at lib/privs.c:647
#6 0x0000ffffa97a03a4 in _zprivs_lower (privs=0xffffa90262e8) at lib/privs.c:727
#7 0x0000aaaaca9565b4 in netlink_talk_info (filter=0xaaaaca954b40 <netlink_talk_filter>, n=0xffffa9028398, dp_info=0xaaab3da829f0, startup=0) at zebra/kernel_netlink.c:976
#8 0x0000aaaaca960b48 in netlink_route_multipath (cmd=24, ctx=0xaaab3da82920) at zebra/rt_netlink.c:1757
#9 0x0000aaaaca960dac in kernel_route_update (ctx=0xaaab3da82920) at zebra/rt_netlink.c:1854
#10 0x0000aaaaca96d770 in kernel_dplane_route_update (ctx=0xaaab3da82920) at zebra/zebra_dplane.c:1751
#11 0x0000aaaaca96d848 in kernel_dplane_process_func (prov=0xaaab09173bc0) at zebra/zebra_dplane.c:1788
#12 0x0000aaaaca96e008 in dplane_thread_loop (event=0xffffa902a870) at zebra/zebra_dplane.c:2184
#13 0x0000ffffa97c1100 in thread_call (thread=0xffffa902a870) at lib/thread.c:1607
#14 0x0000ffffa976c13c in fpt_run (arg=0xaaab092a21f0) at lib/frr_pthread.c:269
#15 0x0000ffffa95047e4 in start_thread (arg=0xffffedb1a62f) at pthread_create.c:486
#16 0x0000ffffa945a98c in thread_start () at ../sysdeps/unix/sysv/linux/aarch64/clone.S:78
(gdb)