Re: [PATCH userspace] libsepol/cil: fix signed overflow caused by using (1 << 31) - 1

[Petr Lautrbach <plautrba@redhat.com>]

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On Wed, Oct 14, 2020 at 08:01:06PM +0200, Petr Lautrbach wrote:
> On Sat, Oct 03, 2020 at 03:19:08PM +0200, Nicolas Iooss wrote:
> > When compiling SELinux userspace tools with -ftrapv (this option
> > generates traps for signed overflow on addition, subtraction,
> > multiplication operations, instead of silently wrapping around),
> > semodule crashes when running the tests from
> > scripts/ci/fedora-test-runner.sh in a Fedora 32 virtual machine:
> > 
> >     [root@localhost selinux-testsuite]# make test
> >     make -C policy load
> >     make[1]: Entering directory '/root/selinux-testsuite/policy'
> >     # Test for "expand-check = 0" in /etc/selinux/semanage.conf
> >     # General policy build
> >     make[2]: Entering directory '/root/selinux-testsuite/policy/test_policy'
> >     Compiling targeted test_policy module
> >     Creating targeted test_policy.pp policy package
> >     rm tmp/test_policy.mod.fc
> >     make[2]: Leaving directory '/root/selinux-testsuite/policy/test_policy'
> >     # General policy load
> >     domain_fd_use --> off
> >     /usr/sbin/semodule -i test_policy/test_policy.pp test_mlsconstrain.cil test_overlay_defaultrange.cil test_add_levels.cil test_glblub.cil
> >     make[1]: *** [Makefile:174: load] Aborted (core dumped)
> > 
> > Using "coredumpctl gdb" leads to the following strack trace:
> > 
> >     (gdb) bt
> >     #0  0x00007f608fe4fa25 in raise () from /lib64/libc.so.6
> >     #1  0x00007f608fe38895 in abort () from /lib64/libc.so.6
> >     #2  0x00007f6090028aca in __addvsi3.cold () from /lib64/libsepol.so.1
> >     #3  0x00007f6090096f59 in __avrule_xperm_setrangebits (low=30, high=30, xperms=0x8b9eea0)
> >         at ../cil/src/cil_binary.c:1551
> >     #4  0x00007f60900970dd in __cil_permx_bitmap_to_sepol_xperms_list (xperms=0xb650a30, xperms_list=0x7ffce2653b18)
> >         at ../cil/src/cil_binary.c:1596
> >     #5  0x00007f6090097286 in __cil_avrulex_ioctl_to_policydb (k=0xb8ec200 "@\023\214\022\006", datum=0xb650a30,
> >         args=0x239a640) at ../cil/src/cil_binary.c:1649
> >     #6  0x00007f609003f1e5 in hashtab_map (h=0x41f8710, apply=0x7f60900971da <__cil_avrulex_ioctl_to_policydb>,
> >         args=0x239a640) at hashtab.c:234
> >     #7  0x00007f609009ea19 in cil_binary_create_allocated_pdb (db=0x2394f10, policydb=0x239a640)
> >         at ../cil/src/cil_binary.c:4969
> >     #8  0x00007f609009d19d in cil_binary_create (db=0x2394f10, policydb=0x7ffce2653d30) at ../cil/src/cil_binary.c:4329
> >     #9  0x00007f609008ec23 in cil_build_policydb_create_pdb (db=0x2394f10, sepol_db=0x7ffce2653d30)
> >         at ../cil/src/cil.c:631
> >     #10 0x00007f608fff4bf3 in semanage_direct_commit () from /lib64/libsemanage.so.1
> >     #11 0x00007f608fff9fae in semanage_commit () from /lib64/libsemanage.so.1
> >     #12 0x0000000000403e2b in main (argc=7, argv=0x7ffce2655058) at semodule.c:753
> > 
> >     (gdb) f 3
> >     #3  0x00007f6090096f59 in __avrule_xperm_setrangebits (low=30, high=30, xperms=0x8b9eea0)
> >         at ../cil/src/cil_binary.c:1551
> >     1551     xperms->perms[i] |= XPERM_SETBITS(h) - XPERM_SETBITS(low);
> > 
> > A signed integer overflow therefore occurs in XPERM_SETBITS(h):
> > 
> >     #define XPERM_SETBITS(x) ((1 << (x & 0x1f)) - 1)
> > 
> > This macro is expanded with h=31, so "(1 << 31) - 1" is computed:
> > 
> > * (1 << 31) = -0x80000000 is the lowest signed 32-bit integer value
> > * (1 << 31) - 1 overflows the capacity of a signed 32-bit integer and
> >   results in 0x7fffffff (which is unsigned)
> > 
> > Using unsigned integers (with "1U") fixes the crash, as
> > (1U << 31) = 0x80000000U has no overflowing issues.
> > 
> > Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>
> 
> Acked-by: Petr Lautrbach <plautrba@redhat.com>

Merged, thanks!

> > ---
> >  checkpolicy/policy_define.c   | 2 +-
> >  libsepol/cil/src/cil_binary.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/checkpolicy/policy_define.c b/checkpolicy/policy_define.c
> > index 395f62284e3c..bf6c3e68bef3 100644
> > --- a/checkpolicy/policy_define.c
> > +++ b/checkpolicy/policy_define.c
> > @@ -2147,7 +2147,7 @@ out:
> >  /* index of the u32 containing the permission */
> >  #define XPERM_IDX(x) (x >> 5)
> >  /* set bits 0 through x-1 within the u32 */
> > -#define XPERM_SETBITS(x) ((1 << (x & 0x1f)) - 1)
> > +#define XPERM_SETBITS(x) ((1U << (x & 0x1f)) - 1)
> >  /* low value for this u32 */
> >  #define XPERM_LOW(x) (x << 5)
> >  /* high value for this u32 */
> > diff --git a/libsepol/cil/src/cil_binary.c b/libsepol/cil/src/cil_binary.c
> > index 36720eda4549..e417c5c28b8b 100644
> > --- a/libsepol/cil/src/cil_binary.c
> > +++ b/libsepol/cil/src/cil_binary.c
> > @@ -1526,7 +1526,7 @@ int cil_avrule_to_policydb(policydb_t *pdb, const struct cil_db *db, struct cil_
> >  /* index of the u32 containing the permission */
> >  #define XPERM_IDX(x) (x >> 5)
> >  /* set bits 0 through x-1 within the u32 */
> > -#define XPERM_SETBITS(x) ((1 << (x & 0x1f)) - 1)
> > +#define XPERM_SETBITS(x) ((1U << (x & 0x1f)) - 1)
> >  /* low value for this u32 */
> >  #define XPERM_LOW(x) (x << 5)
> >  /* high value for this u32 */
> > -- 
> > 2.28.0
> > 
> 

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