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From: Florian Weimer <fweimer@redhat.com>
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Date: Tue, 21 Jan 2020 16:38:15 +0000 (+0100)
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Subject: resolv: Fix ABA race in /etc/resolv.conf change detection [BZ #25420]
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X-Git-Url: https://sourceware.org/git/?p=glibc.git;a=commitdiff_plain;h=fa00db0a6eb755837ae5d413515e0da582b304f3
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resolv: Fix ABA race in /etc/resolv.conf change detection [BZ #25420]
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__resolv_conf_get_current should only record the initial file
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change data if after verifying that file just read matches the
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original measurement. Fixes commit aef16cc8a4c670036d45590877
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("resolv: Automatically reload a changed /etc/resolv.conf file
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[BZ #984]").
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Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
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---
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diff --git a/resolv/resolv_conf.c b/resolv/resolv_conf.c
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index bdd2ebb909..29a1f4fb94 100644
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--- a/resolv/resolv_conf.c
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+++ b/resolv/resolv_conf.c
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@@ -136,18 +136,25 @@ __resolv_conf_get_current (void)
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{
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/* Parse configuration while holding the lock. This avoids
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duplicate work. */
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- conf = __resolv_conf_load (NULL, NULL);
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+ struct file_change_detection after_load;
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+ conf = __resolv_conf_load (NULL, &after_load);
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if (conf != NULL)
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{
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if (global_copy->conf_current != NULL)
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conf_decrement (global_copy->conf_current);
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global_copy->conf_current = conf; /* Takes ownership. */
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- /* Update file modification stamps. The configuration we
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- read could be a newer version of the file, but this does
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- not matter because this will lead to an extraneous reload
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- later. */
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- global_copy->file_resolve_conf = initial;
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+ /* Update file change detection data, but only if it matches
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+ the initial measurement. This avoids an ABA race in case
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+ /etc/resolv.conf is temporarily replaced while the file
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+ is read (after the initial measurement), and restored to
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+ the initial version later. */
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+ if (file_is_unchanged (&initial, &after_load))
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+ global_copy->file_resolve_conf = after_load;
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+ else
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+ /* If there is a discrepancy, trigger a reload during the
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+ next use. */
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+ global_copy->file_resolve_conf.size = -1;
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}
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}
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