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Fix PAM response allocation to handle invalid message count #281

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Fix PAM response allocation to handle invalid message count #281

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9 changes: 8 additions & 1 deletion src/auth/passdb-pam.c
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Original file line number Diff line number Diff line change
Expand Up @@ -113,7 +113,14 @@ pam_userpass_conv(int num_msg, pam_const struct pam_message **msg,

*resp_r = NULL;

resp = calloc(num_msg, sizeof(struct pam_response));
if (num_msg < 0 ||
(size_t)num_msg > SIZE_MAX / sizeof(struct pam_response)) {
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@sirainen sirainen Apr 30, 2026

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calloc() already checks for integer overflows, and returns NULL if there would be an overflow. And Dovecot handles NULL already by failing with out-of-memory. Other large num_msgs are similarly handled as NULL -> out-of-memory. I guess there's the question of whether we should enforce some sane limit, but since PAM is a very trusted source I don't think it makes practically any difference.

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@netliomax25-code netliomax25-code May 2, 2026

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You're right that calloc() already handles overflow and that the NULL
path is properly handled here, so this doesn’t introduce a practical
issue as-is.

Looking at it again, the more relevant concern here would be avoiding
excessive allocations if num_msg is unexpectedly large. Even if PAM is
generally trusted, adding a reasonable upper bound could make this path
more robust and easier to reason about.

I can update this PR to add a limit check instead, or drop the change
if you'd prefer to keep the current behavior.

e_error(authdb_event(ctx->request),
"pam: invalid response count: %d", num_msg);
return PAM_CONV_ERR;
}

resp = calloc((size_t)num_msg, sizeof(struct pam_response));
if (resp == NULL)
i_fatal_status(FATAL_OUTOFMEM, "Out of memory");

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