wp_seed_src: read /dev/urandom via raw fd to avoid seccomp SIGSYS#440
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ColtonWilley wants to merge 1 commit into
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wp_seed_src: read /dev/urandom via raw fd to avoid seccomp SIGSYS#440ColtonWilley wants to merge 1 commit into
ColtonWilley wants to merge 1 commit into
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SEED-SRC opened /dev/urandom with XFOPEN (buffered fopen). The first fread pulls read-ahead into the stdio buffer. sshd primes its RNG before the privsep fork, so the preauth child inherits that buffered FILE*; at child exit glibc stdio cleanup calls lseek() to drop the unread read-ahead. lseek() isn't in OpenSSH's preauth seccomp filter, so the child is killed with SIGSYS. Use a raw fd (open/read/close, O_CLOEXEC) instead: no buffer, no read-ahead, no exit-time lseek, matching OpenSSL's own device seeding. The fd and wolfSSL seed callback are shared and reference counted across provider contexts; the last unload closes the fd and restores the seed callback to wc_GenerateSeed (not NULL, which fails wc_InitRng with DRBG_NO_SEED_CB under FIPS). Also fixes a re-init leak that orphaned the prior buffered handle.
padelsbach
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Jul 10, 2026
padelsbach
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padelsbach
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Good improvement. Couple nits
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| (void)os; | ||
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| if (WP_URANDOM_LOCK() != 0) { |
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In addition to the lock, should probably check that the refcount >= 1 just to make sure the file is still open
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| if (g_urandom_fd == XBADFD) { | ||
| do { | ||
| g_urandom_fd = open(URANDOM_PATH, O_RDONLY | O_CLOEXEC); |
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Can we use XOPEN here for consistency?
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Problem
SEED-SRC read
/dev/urandomthrough a buffered stdioFILE*(XFOPEN). That breaks OpenSSH's privsep preauth child:sshdprimes its RNG (seed_rng->RAND_status) before the privsep fork, so the firstfreadpulls a block of read-ahead into the stdio buffer.FILE*and its fd.lseek()to discard the unread read-ahead.lseek()is not in OpenSSH's preauth seccomp allow-list, so the child is killed withSIGSYS(audit showssig=31 syscall=8).Stock OpenSSL reads the device with a raw fd and never hits this; the buffered
FILE*was the defect. Separately, re-initializing SEED-SRC (a second provider load) orphaned the previous buffered handle.Fix
/dev/urandomwith a raw fd (open/read/close,O_CLOEXEC). No stdio buffer, so no read-ahead and no exit-timelseek.wc_GenerateSeed— notNULL, which would make a laterwc_InitRngfailDRBG_NO_SEED_CBunder FIPS.NO_WOLFSSL_DIRbuilds#errorrather than silently falling back.Tests
test_seccomp_sandbox— runs under the verbatim OpenSSH 9.9p1 preauth filter: single-context (the original crash), re-init, and multi-context survivor. Compiled in with-DWP_TEST_SECCOMP_SANDBOX(enabled by theseed-srcCI job; a skip stub otherwise, since it needsPR_SET_SECCOMP).test_seed_src_refcount— multi-context refcount; the first unload must not close the shared fd.test_seed_src_reload— drives the refcount to zero (fd close + seed-callback restore) and reloads; re-exec'd so the count actually reaches zero, which the main suite's own reference otherwise prevents.