SIGPIPE from ssh-keyscan [patch]

[Jim Thompson]

The remote closed the session for some reason before ssh-keyscan wrote the greening ("SSH-2.0-OpenSSH-keyscan\r\n”), so you got SIGPIPE and ERRNO = 32 back from the write call.

Arguably the right thing occurred here, with the exception that it killed your ssh-keyscan process.

So perhaps instead of ignoring the signal, you should find out why the remote is exiting before the local can send its greeting.

Otherwise, it’s a bit less heavy-handed to 

Int set = 1;
setsockopt(sd, SOL_SOCKET, SO_NOSIGPIPE, (void *)&set, sizeof(int));

Where sd is the descriptor in question (16 in your example below).

But other parts of ssh-keyscan seem to want to know that EPIPE has occurred, so neither is the correction solution here.

Jim


> On May 1, 2019, at 5:05 PM, Alan Amesbury <amesbury at oitsec.umn.edu> wrote:
> 
> The stock ssh-keyscan bundled with 12.0-RELEASE exits with a SIGPIPE when it receives weird behavior from hosts it's attempting to communicate with.  Symptoms look like:
> 
> 
> % ssh-keyscan -f /tmp/randtargetlist > /dev/null
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> Broken pipe
> % 
> 
> 
> Output from truss confirms it's SIGPIPE:
> 
> 			.
> 			.
> 			.
> 99597: write(7,"\0\0\^Dd\a\^T\M-Y\M-Jw(E\M-ty"...,1128) = 1128 (0x468)
> 99597: select(8,{ 7 },0x0,0x0,{ 5.000000 })      = 1 (0x1)
> 99597: read(7,"\0\0\^D\M-|\n\^T\M^X\M-N]\M-O\^C"...,8192) = 1280 (0x500)
> 99597: write(7,"\0\0\0,\^F\^^\0\0\0 0\M^S\M^J#"...,48) = 48 (0x30)
> 99597: select(8,{ 7 },0x0,0x0,{ 5.000000 })      = 1 (0x1)
> 99597: read(7,"\0\0\0\M-<\b\^_\0\0\0003\0\0\0\v"...,8192) = 208 (0xd0)
> 99597: write(1,"[REDACTED] ssh-ed255"...,104) = 104 (0x68)
> 99597: close(7)                                  = 0 (0x0)
> 99597: write(16,"SSH-2.0-OpenSSH-keyscan\r\n",25) ERR#32 'Broken pipe'
> 99597: process killed, signal = 13
> 
> 
> 
> The behavior exists in openssh-portable ("$FreeBSD: head/security/openssh-portable/Makefile 484842 2018-11-12 21:55:35Z bdrewery $") as well.
> 
> The arguably naive patch I came up with is:
> 
> 
> --- /tmp/ssh-keyscan.c	2019-05-01 16:09:11.761587000 -0500
> +++ ssh-keyscan.c	2019-05-01 16:08:50.425879000 -0500
> @@ -644,6 +644,8 @@
> int
> main(int argc, char **argv)
> {
> +        // ignore SIGPIPE
> +        signal(SIGPIPE, SIG_IGN);
> 	int debug_flag = 0, log_level = SYSLOG_LEVEL_INFO;
> 	int opt, fopt_count = 0, j;
> 	char *tname, *cp, *line = NULL;
> 
> 
> 
> 
> Straightforward and brutish:  it ignores SIGPIPE within the main function in ssh-keyscan.c.  This appears to work as expected, e.g.:
> 
> 
> % ./ssh-keyscan_PATCHED -f /tmp/randtargetlist -T 15 > /dev/null
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> # [REDACTED]:22 SSH-2.0-OpenSSH_7.4
> write ([REDACTED]): Broken pipe
> write ([REDACTED]): Broken pipe
> write ([REDACTED]): Broken pipe
> # [REDACTED]:22 SSH-2.0-babeld-81e0741
> 			.
> 			.
> 			.
> 
> 
> 
> Is this something that's best done by adding it upstream, in the FreeBSD source (and ports), or ???  Also, is this sane?  I don't see it as a huge deal because it's not a modification to the actual server or client code, just to the part that grabs host keys, but I freely admit that I'm outta my depth here.
> 
> 
> -- 
> Alan
> 
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