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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
change data if after verifying that file just read matches the
original measurement.  Fixes commit aef16cc8a4
("resolv: Automatically reload a changed /etc/resolv.conf file
[BZ #984]").

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
This commit is contained in:
Florian Weimer 2020-01-21 17:38:15 +01:00
parent dd0b4df329
commit fa00db0a6e
1 changed files with 13 additions and 6 deletions
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19
resolv/resolv_conf.c
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@ -136,18 +136,25 @@ __resolv_conf_get_current (void)
{ {
/* Parse configuration while holding the lock. This avoids /* Parse configuration while holding the lock. This avoids
duplicate work. */ duplicate work. */
conf = __resolv_conf_load (NULL, NULL); struct file_change_detection after_load;
conf = __resolv_conf_load (NULL, &after_load);
if (conf != NULL) if (conf != NULL)
{ {
if (global_copy->conf_current != NULL) if (global_copy->conf_current != NULL)
conf_decrement (global_copy->conf_current); conf_decrement (global_copy->conf_current);
global_copy->conf_current = conf; /* Takes ownership. */ global_copy->conf_current = conf; /* Takes ownership. */
/* Update file modification stamps. The configuration we /* Update file change detection data, but only if it matches
read could be a newer version of the file, but this does the initial measurement. This avoids an ABA race in case
not matter because this will lead to an extraneous reload /etc/resolv.conf is temporarily replaced while the file
later. */ is read (after the initial measurement), and restored to
global_copy->file_resolve_conf = initial; the initial version later. */
if (file_is_unchanged (&initial, &after_load))
global_copy->file_resolve_conf = after_load;
else
/* If there is a discrepancy, trigger a reload during the
next use. */
global_copy->file_resolve_conf.size = -1;
} }
} }