6.10-stable review patch. If anyone has any objections, please let me know.
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From: Johannes Berg johannes.berg@intel.com
[ Upstream commit 2cf3a3c4b84def5406b830452b1cb8bbfffe0ebe ]
When signals are hard-blocked in order to do time-travel socket processing, we set signals_blocked and then handle SIGIO signals by setting the SIGIO bit in signals_pending. When unblocking, we first set signals_blocked to 0, and then handle all pending signals. We have to set it first, so that we can again properly block/unblock inside the unblock, if the time-travel handlers need to be processed.
Unfortunately, this is racy. We can get into this situation:
// signals_pending = SIGIO_MASK
unblock_signals_hard() signals_blocked = 0; if (signals_pending && signals_enabled) { block_signals(); unblock_signals() ... sig_handler_common(SIGIO, NULL, NULL); sigio_handler() ... sigio_reg_handler() irq_do_timetravel_handler() reg->timetravel_handler() == vu_req_interrupt_comm_handler() vu_req_read_message() vhost_user_recv_req() vhost_user_recv() vhost_user_recv_header() // reads 12 bytes header of // 20 bytes message <-- receive SIGIO here <-- sig_handler() int enabled = signals_enabled; // 1 if ((signals_blocked || !enabled) && (sig == SIGIO)) { if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL) sigio_run_timetravel_handlers() _sigio_handler() sigio_reg_handler() ... as above ... vhost_user_recv_header() // reads 8 bytes that were message payload // as if it were header - but aborts since // it then gets -EAGAIN ... --> end signal handler --> // continue in vhost_user_recv() // full_read() for 8 bytes payload busy loops // entire process hangs here
Conceptually, to fix this, we need to ensure that the signal handler cannot run while we hard-unblock signals. The thing that makes this more complex is that we can be doing hard-block/unblock while unblocking. Introduce a new signals_blocked_pending variable that we can keep at non-zero as long as pending signals are being processed, then we only need to ensure it's decremented safely and the signal handler will only increment it if it's already non-zero (or signals_blocked is set, of course.)
Note also that only the outermost call to hard-unblock is allowed to decrement signals_blocked_pending, since it could otherwise reach zero in an inner call, and leave the same race happening if the timetravel_handler loops, but that's basically required of it.
Fixes: d6b399a0e02a ("um: time-travel/signals: fix ndelay() in interrupt") Link: https://patch.msgid.link/20240703110144.28034-2-johannes@sipsolutions.net Signed-off-by: Johannes Berg johannes.berg@intel.com Signed-off-by: Sasha Levin sashal@kernel.org --- arch/um/os-Linux/signal.c | 118 +++++++++++++++++++++++++++++++------- 1 file changed, 98 insertions(+), 20 deletions(-)
diff --git a/arch/um/os-Linux/signal.c b/arch/um/os-Linux/signal.c index 787cfb9a03088..b11ed66c8bb0e 100644 --- a/arch/um/os-Linux/signal.c +++ b/arch/um/os-Linux/signal.c @@ -8,6 +8,7 @@
#include <stdlib.h> #include <stdarg.h> +#include <stdbool.h> #include <errno.h> #include <signal.h> #include <string.h> @@ -65,9 +66,7 @@ static void sig_handler_common(int sig, struct siginfo *si, mcontext_t *mc)
int signals_enabled; #ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT -static int signals_blocked; -#else -#define signals_blocked 0 +static int signals_blocked, signals_blocked_pending; #endif static unsigned int signals_pending; static unsigned int signals_active = 0; @@ -76,14 +75,27 @@ static void sig_handler(int sig, struct siginfo *si, mcontext_t *mc) { int enabled = signals_enabled;
- if ((signals_blocked || !enabled) && (sig == SIGIO)) { +#ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT + if ((signals_blocked || + __atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) && + (sig == SIGIO)) { + /* increment so unblock will do another round */ + __atomic_add_fetch(&signals_blocked_pending, 1, + __ATOMIC_SEQ_CST); + return; + } +#endif + + if (!enabled && (sig == SIGIO)) { /* * In TT_MODE_EXTERNAL, need to still call time-travel - * handlers unless signals are also blocked for the - * external time message processing. This will mark - * signals_pending by itself (only if necessary.) + * handlers. This will mark signals_pending by itself + * (only if necessary.) + * Note we won't get here if signals are hard-blocked + * (which is handled above), in that case the hard- + * unblock will handle things. */ - if (!signals_blocked && time_travel_mode == TT_MODE_EXTERNAL) + if (time_travel_mode == TT_MODE_EXTERNAL) sigio_run_timetravel_handlers(); else signals_pending |= SIGIO_MASK; @@ -380,33 +392,99 @@ int um_set_signals_trace(int enable) #ifdef UML_CONFIG_UML_TIME_TRAVEL_SUPPORT void mark_sigio_pending(void) { + /* + * It would seem that this should be atomic so + * it isn't a read-modify-write with a signal + * that could happen in the middle, losing the + * value set by the signal. + * + * However, this function is only called when in + * time-travel=ext simulation mode, in which case + * the only signal ever pending is SIGIO, which + * is blocked while this can be called, and the + * timer signal (SIGALRM) cannot happen. + */ signals_pending |= SIGIO_MASK; }
void block_signals_hard(void) { - if (signals_blocked) - return; - signals_blocked = 1; + signals_blocked++; barrier(); }
void unblock_signals_hard(void) { + static bool unblocking; + if (!signals_blocked) + panic("unblocking signals while not blocked"); + + if (--signals_blocked) return; - /* Must be set to 0 before we check the pending bits etc. */ - signals_blocked = 0; + /* + * Must be set to 0 before we check pending so the + * SIGIO handler will run as normal unless we're still + * going to process signals_blocked_pending. + */ barrier();
- if (signals_pending && signals_enabled) { - /* this is a bit inefficient, but that's not really important */ - block_signals(); - unblock_signals(); - } else if (signals_pending & SIGIO_MASK) { - /* we need to run time-travel handlers even if not enabled */ - sigio_run_timetravel_handlers(); + /* + * Note that block_signals_hard()/unblock_signals_hard() can be called + * within the unblock_signals()/sigio_run_timetravel_handlers() below. + * This would still be prone to race conditions since it's actually a + * call _within_ e.g. vu_req_read_message(), where we observed this + * issue, which loops. Thus, if the inner call handles the recorded + * pending signals, we can get out of the inner call with the real + * signal hander no longer blocked, and still have a race. Thus don't + * handle unblocking in the inner call, if it happens, but only in + * the outermost call - 'unblocking' serves as an ownership for the + * signals_blocked_pending decrement. + */ + if (unblocking) + return; + unblocking = true; + + while (__atomic_load_n(&signals_blocked_pending, __ATOMIC_SEQ_CST)) { + if (signals_enabled) { + /* signals are enabled so we can touch this */ + signals_pending |= SIGIO_MASK; + /* + * this is a bit inefficient, but that's + * not really important + */ + block_signals(); + unblock_signals(); + } else { + /* + * we need to run time-travel handlers even + * if not enabled + */ + sigio_run_timetravel_handlers(); + } + + /* + * The decrement of signals_blocked_pending must be atomic so + * that the signal handler will either happen before or after + * the decrement, not during a read-modify-write: + * - If it happens before, it can increment it and we'll + * decrement it and do another round in the loop. + * - If it happens after it'll see 0 for both signals_blocked + * and signals_blocked_pending and thus run the handler as + * usual (subject to signals_enabled, but that's unrelated.) + * + * Note that a call to unblock_signals_hard() within the calls + * to unblock_signals() or sigio_run_timetravel_handlers() above + * will do nothing due to the 'unblocking' state, so this cannot + * underflow as the only one decrementing will be the outermost + * one. + */ + if (__atomic_sub_fetch(&signals_blocked_pending, 1, + __ATOMIC_SEQ_CST) < 0) + panic("signals_blocked_pending underflow"); } + + unblocking = false; } #endif