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

[Petr Lautrbach <plautrba@redhat.com>]

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>

> ---
>  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
>