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cred: crextend(): Harden, simplify

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Harden by adding more assertions, and a plain panic in case of an
unrepresentable size for the groups array (this can never happen after
the change of the 'kern.ngroups' computation to impose some not too high
maximum value a few commits ago).  Fix an impact in kern_setgroups().

Simplify by removing the iterative process whose purpose is actually to
determine the closest power of two that is greater than the wanted
number of bytes.  Using the proper target quantity (number of bytes)
incidentally helps with eliminating divisions (and the reliance on
sizeof(gid_t) being a power of two).

Reviewed by:    mhorne (older version)
Approved by:    markj (mentor)
MFC after:      3 days
Differential Revision:  https://reviews.freebsd.org/D46915
This commit is contained in:
Olivier Certner 2024-10-01 16:37:11 +02:00
parent 6346750678
commit ea26c0e797
No known key found for this signature in database
GPG key ID: 8CA13040971E2627
2 changed files with 39 additions and 24 deletions
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57
sys/kern/kern_prot.c
View file

@ -888,7 +888,8 @@ kern_setgroups(struct thread *td, int *ngrpp, gid_t *groups)
ngrp = *ngrpp;
}
newcred = crget();
crextend(newcred, ngrp);
if (ngrp != 0)
crextend(newcred, ngrp);
PROC_LOCK(p);
oldcred = crcopysafe(p, newcred);
@ -2226,6 +2227,13 @@ crcopy(struct ucred *dest, struct ucred *src)
bcopy(&src->cr_startcopy, &dest->cr_startcopy,
(unsigned)((caddr_t)&src->cr_endcopy -
(caddr_t)&src->cr_startcopy));
/*
* Avoids an assertion in crsetgroups() -> crextend(). Ideally,
* 'cr_ngroups' should be moved out of 'struct ucred''s bcopied area,
* but this would break the ABI, so is deferred until there is a real
* need to change the ABI.
*/
dest->cr_ngroups = 0;
dest->cr_flags = src->cr_flags;
crsetgroups(dest, src->cr_ngroups, src->cr_groups);
uihold(dest->cr_uidinfo);
@ -2350,45 +2358,48 @@ crcopysafe(struct proc *p, struct ucred *cr)
}
/*
* Extend the passed in credential to hold n items.
* Extend the passed-in credentials to hold n groups.
*
* Must not be called after groups have been set.
*/
void
crextend(struct ucred *cr, int n)
{
int cnt;
size_t nbytes;
MPASS2(cr->cr_ref == 1, "'cr_ref' must be 1 (referenced, unshared)");
MPASS2(cr->cr_ngroups == 0, "groups on 'cr' already set!");
groups_check_positive_len(n);
groups_check_max_len(n);
/* Truncate? */
if (n <= cr->cr_agroups)
return;
/*
* We extend by 2 each time since we're using a power of two
* allocator until we need enough groups to fill a page.
* Once we're allocating multiple pages, only allocate as many
* as we actually need. The case of processes needing a
* non-power of two number of pages seems more likely than
* a real world process that adds thousands of groups one at a
* time.
*/
if ( n < PAGE_SIZE / sizeof(gid_t) ) {
if (cr->cr_agroups == 0)
cnt = MAX(1, MINALLOCSIZE / sizeof(gid_t));
else
cnt = cr->cr_agroups * 2;
nbytes = n * sizeof(gid_t);
if (nbytes < n)
panic("Too many groups (memory size overflow)! "
"Computation of 'kern.ngroups' should have prevented this, "
"please fix it. In the meantime, reduce 'kern.ngroups'.");
while (cnt < n)
cnt *= 2;
/*
* We allocate a power of 2 larger than 'nbytes', except when that
* exceeds PAGE_SIZE, in which case we allocate the right multiple of
* pages. We assume PAGE_SIZE is a power of 2 (the call to roundup2()
* below) but do not need to for sizeof(gid_t).
*/
if (nbytes < PAGE_SIZE) {
if (!powerof2(nbytes))
/* fls*() return a bit index starting at 1. */
nbytes = 1 << flsl(nbytes);
} else
cnt = roundup2(n, PAGE_SIZE / sizeof(gid_t));
nbytes = roundup2(nbytes, PAGE_SIZE);
/* Free the old array. */
if (cr->cr_groups != cr->cr_smallgroups)
free(cr->cr_groups, M_CRED);
cr->cr_groups = malloc(cnt * sizeof(gid_t), M_CRED, M_WAITOK | M_ZERO);
cr->cr_agroups = cnt;
cr->cr_groups = malloc(nbytes, M_CRED, M_WAITOK | M_ZERO);
cr->cr_agroups = nbytes / sizeof(gid_t);
}
/*

6
sys/sys/ucred.h
View file

@ -73,6 +73,10 @@ struct ucred {
uid_t cr_uid; /* effective user id */
uid_t cr_ruid; /* real user id */
uid_t cr_svuid; /* saved user id */
/*
* XXXOC: On the next ABI change, please move 'cr_ngroups' out of the
* copied area (crcopy() already copes with this change).
*/
int cr_ngroups; /* number of groups */
gid_t cr_rgid; /* real group id */
gid_t cr_svgid; /* saved group id */
@ -80,7 +84,7 @@ struct ucred {
struct uidinfo *cr_ruidinfo; /* per ruid resource consumption */
struct prison *cr_prison; /* jail(2) */
struct loginclass *cr_loginclass; /* login class */
void *cr_pspare2[2]; /* general use 2 */
void *cr_pspare2[2]; /* general use 2 */
#define cr_endcopy cr_label
struct label *cr_label; /* MAC label */
gid_t *cr_groups; /* groups */