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VM: fix race condition communicating with VM

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After processing certain asynchronous requests from VFS, VM would send
an asynchronous reply without supplying the AMF_NOREPLY flag.  As a
result, this asynchronous reply could be taken as the result of an
ipc_sendrec() call, causing the entire VM/VFS communication to become
desynchronized.  The end result was a deadlock-induced panic during a
later request.

This bug was exposed because of the higher-than-usual concurrency
level in the NetBSD rc scripts.  The fix consists of properly setting
the AMF_NOREPLY flag for asynchronous replies.

Change-Id: Iafafe2fdd67f212ecbf27a53862cefba2e4cf7e8
This commit is contained in:
David van Moolenbroek 2017-02-23 11:20:33 +00:00
parent 3e1f70db42
commit 2109df2759
1 changed files with 9 additions and 3 deletions
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12
minix/servers/vm/pagefaults.c
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@ -197,7 +197,7 @@ static void handle_memory_continue(struct vmproc *vmp, message *m,
static void handle_memory_final(struct hm_state *state, int result) static void handle_memory_final(struct hm_state *state, int result)
{ {
int r; int r, flag;
assert(state); assert(state);
assert(state->valid == VALID); assert(state->valid == VALID);
@ -215,9 +215,15 @@ static void handle_memory_final(struct hm_state *state, int result)
assert(state->caller == VFS_PROC_NR); assert(state->caller == VFS_PROC_NR);
/* If a transaction ID was set, reset it */ /* If a transaction ID was set, reset it */
msg.m_type = TRNS_ADD_ID(msg.m_type, state->transid); msg.m_type = TRNS_ADD_ID(msg.m_type, state->transid);
} flag = AMF_NOREPLY;
} else
flag = 0;
if(asynsend3(state->caller, &msg, 0) != OK) { /*
* Use AMF_NOREPLY only if there was a transaction ID, which
* signifies that VFS issued the request asynchronously.
*/
if(asynsend3(state->caller, &msg, flag) != OK) {
panic("handle_memory_final: asynsend3 failed"); panic("handle_memory_final: asynsend3 failed");
} }