diff mbox series

[bpf-next,1/2] bpf: add bound tracking for BPF_MOD

Message ID 20230306033119.2634976-2-xukuohai@huaweicloud.com
State New
Headers show
Series bpf: add bound tracking for BPF_MOD | expand

Commit Message

Xu Kuohai March 6, 2023, 3:31 a.m. UTC 
From: Xu Kuohai <xukuohai@huawei.com>

dst_reg is marked as unknown when BPF_MOD instruction is verified, causing
the following bpf prog to be incorrectly rejected.

0: r0 = 0
1: r0 %= 10   // r0 is marked as unknown
2: r1 = 0
3: r1 += 1
4: if r1 < r0 goto pc-2 // verifier concludes the loop is unbounded
5: exit

To teach verifier to accept the above prog, this patch adds bound tracking
for BPF_MOD, based on the following observation.

BPF_MOD is unsigned and for a given unsigned divisor x:
1. when x != 0, dst_reg bits are in the range [0, x);
2. when x == 0, dst_reg is truncated to 32 bits by mod32 or remains
   unchanged by mod64.

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
---
 kernel/bpf/verifier.c | 98 ++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 93 insertions(+), 5 deletions(-)

Comments

kernel test robot March 5, 2023, 6 p.m. UTC | #1 
Hi Xu,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/intel-lab-lkp/linux/commits/Xu-Kuohai/bpf-add-bound-tracking-for-BPF_MOD/20230305-223257
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
patch link:    https://lore.kernel.org/r/20230306033119.2634976-2-xukuohai%40huaweicloud.com
patch subject: [PATCH bpf-next 1/2] bpf: add bound tracking for BPF_MOD
config: arm-randconfig-r025-20230305 (https://download.01.org/0day-ci/archive/20230306/202303060155.cNDEo1Br-lkp@intel.com/config)
compiler: clang version 17.0.0 (https://github.com/llvm/llvm-project 67409911353323ca5edf2049ef0df54132fa1ca7)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install arm cross compiling tool for clang build
        # apt-get install binutils-arm-linux-gnueabi
        # https://github.com/intel-lab-lkp/linux/commit/e66c7bbd32e375af92c776a2b9f51be4c515ad71
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Xu-Kuohai/bpf-add-bound-tracking-for-BPF_MOD/20230305-223257
        git checkout e66c7bbd32e375af92c776a2b9f51be4c515ad71
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm olddefconfig
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross W=1 O=build_dir ARCH=arm SHELL=/bin/bash kernel/

If you fix the issue, kindly add following tag where applicable
| Reported-by: kernel test robot <lkp@intel.com>
| Link: https://lore.kernel.org/oe-kbuild-all/202303060155.cNDEo1Br-lkp@intel.com/

All errors (new ones prefixed by >>):

   kernel/bpf/verifier.c:10298:24: warning: array index 16 is past the end of the array (that has type 'u32[16]' (aka 'unsigned int[16]')) [-Warray-bounds]
                                      meta.func_id == special_kfunc_list[KF_bpf_dynptr_slice_rdwr]) {
                                                      ^                  ~~~~~~~~~~~~~~~~~~~~~~~~
   kernel/bpf/verifier.c:9150:1: note: array 'special_kfunc_list' declared here
   BTF_ID_LIST(special_kfunc_list)
   ^
   include/linux/btf_ids.h:207:27: note: expanded from macro 'BTF_ID_LIST'
   #define BTF_ID_LIST(name) static u32 __maybe_unused name[16];
                             ^
   kernel/bpf/verifier.c:11622:13: warning: comparison of distinct pointer types ('typeof ((umax)) *' (aka 'unsigned int *') and 'uint64_t *' (aka 'unsigned long long *')) [-Wcompare-distinct-pointer-types]
           umax_rem = do_div(umax, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:222:28: note: expanded from macro 'do_div'
           (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
                  ~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~
>> kernel/bpf/verifier.c:11622:13: error: incompatible pointer types passing 'u32 *' (aka 'unsigned int *') to parameter of type 'uint64_t *' (aka 'unsigned long long *') [-Werror,-Wincompatible-pointer-types]
           umax_rem = do_div(umax, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:238:22: note: expanded from macro 'do_div'
                   __rem = __div64_32(&(n), __base);       \
                                      ^~~~
   arch/arm/include/asm/div64.h:24:45: note: passing argument to parameter 'n' here
   static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
                                               ^
   kernel/bpf/verifier.c:11623:13: warning: comparison of distinct pointer types ('typeof ((umin)) *' (aka 'unsigned int *') and 'uint64_t *' (aka 'unsigned long long *')) [-Wcompare-distinct-pointer-types]
           umin_rem = do_div(umin, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:222:28: note: expanded from macro 'do_div'
           (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
                  ~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~
   kernel/bpf/verifier.c:11623:13: error: incompatible pointer types passing 'u32 *' (aka 'unsigned int *') to parameter of type 'uint64_t *' (aka 'unsigned long long *') [-Werror,-Wincompatible-pointer-types]
           umin_rem = do_div(umin, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:238:22: note: expanded from macro 'do_div'
                   __rem = __div64_32(&(n), __base);       \
                                      ^~~~
   arch/arm/include/asm/div64.h:24:45: note: passing argument to parameter 'n' here
   static inline uint32_t __div64_32(uint64_t *n, uint32_t base)
                                               ^
   kernel/bpf/verifier.c:11622:13: warning: shift count >= width of type [-Wshift-count-overflow]
           umax_rem = do_div(umax, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:234:25: note: expanded from macro 'do_div'
           } else if (likely(((n) >> 32) == 0)) {          \
                                  ^  ~~
   include/linux/compiler.h:77:40: note: expanded from macro 'likely'
   # define likely(x)      __builtin_expect(!!(x), 1)
                                               ^
   kernel/bpf/verifier.c:11623:13: warning: shift count >= width of type [-Wshift-count-overflow]
           umin_rem = do_div(umin, val);
                      ^~~~~~~~~~~~~~~~~
   include/asm-generic/div64.h:234:25: note: expanded from macro 'do_div'
           } else if (likely(((n) >> 32) == 0)) {          \
                                  ^  ~~
   include/linux/compiler.h:77:40: note: expanded from macro 'likely'
   # define likely(x)      __builtin_expect(!!(x), 1)
                                               ^
   5 warnings and 2 errors generated.


vim +11622 kernel/bpf/verifier.c

 11606	
 11607	static void scalar32_min_max_mod(struct bpf_reg_state *dst_reg,
 11608					 struct bpf_reg_state *src_reg)
 11609	{
 11610		u32 val = (u32)src_reg->var_off.value; /* src_reg is const */
 11611		u32 umax = dst_reg->u32_max_value;
 11612		u32 umin = dst_reg->u32_min_value;
 11613		u32 umax_rem, umin_rem;
 11614	
 11615		/* dst_reg is 32-bit truncated when mod32 zero, since
 11616		 * adjust_scalar_min_max_vals calls zext_32_to_64 to do truncation for
 11617		 * all alu32 ops, here we do nothing and just return.
 11618		 */
 11619		if (!val)
 11620			return;
 11621	
 11622		umax_rem = do_div(umax, val);
 11623		umin_rem = do_div(umin, val);
 11624	
 11625		/* no winding */
 11626		if (umax - umin < val && umin_rem <= umax_rem) {
 11627			dst_reg->var_off = tnum_range(umin_rem, umax_rem);
 11628			dst_reg->u32_min_value = umin_rem;
 11629			dst_reg->u32_max_value = umax_rem;
 11630		} else {
 11631			dst_reg->var_off = tnum_range(0, val - 1);
 11632			dst_reg->u32_min_value = 0;
 11633			dst_reg->u32_max_value = val - 1;
 11634		}
 11635	
 11636		/* cross the sign boundary */
 11637		if ((s32)dst_reg->u32_min_value > (s32)dst_reg->u32_max_value) {
 11638			dst_reg->s32_min_value = S32_MIN;
 11639			dst_reg->s32_max_value = S32_MAX;
 11640		} else {
 11641			dst_reg->s32_min_value = (s32)dst_reg->u32_min_value;
 11642			dst_reg->s32_max_value = (s32)dst_reg->u32_max_value;
 11643		}
 11644	
 11645		/* mark reg64 unbounded to deduce 64-bit bounds from var_off */
 11646		__mark_reg64_unbounded(dst_reg);
 11647	}
 11648
diff mbox series

Patch

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 272563a0b770..d44a33a53e8e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -11257,6 +11257,87 @@  static void scalar_min_max_arsh(struct bpf_reg_state *dst_reg,
 	__update_reg_bounds(dst_reg);
 }
 
+static void scalar32_min_max_mod(struct bpf_reg_state *dst_reg,
+				 struct bpf_reg_state *src_reg)
+{
+	u32 val = (u32)src_reg->var_off.value; /* src_reg is const */
+	u32 umax = dst_reg->u32_max_value;
+	u32 umin = dst_reg->u32_min_value;
+	u32 umax_rem, umin_rem;
+
+	/* dst_reg is 32-bit truncated when mod32 zero, since
+	 * adjust_scalar_min_max_vals calls zext_32_to_64 to do truncation for
+	 * all alu32 ops, here we do nothing and just return.
+	 */
+	if (!val)
+		return;
+
+	umax_rem = do_div(umax, val);
+	umin_rem = do_div(umin, val);
+
+	/* no winding */
+	if (umax - umin < val && umin_rem <= umax_rem) {
+		dst_reg->var_off = tnum_range(umin_rem, umax_rem);
+		dst_reg->u32_min_value = umin_rem;
+		dst_reg->u32_max_value = umax_rem;
+	} else {
+		dst_reg->var_off = tnum_range(0, val - 1);
+		dst_reg->u32_min_value = 0;
+		dst_reg->u32_max_value = val - 1;
+	}
+
+	/* cross the sign boundary */
+	if ((s32)dst_reg->u32_min_value > (s32)dst_reg->u32_max_value) {
+		dst_reg->s32_min_value = S32_MIN;
+		dst_reg->s32_max_value = S32_MAX;
+	} else {
+		dst_reg->s32_min_value = (s32)dst_reg->u32_min_value;
+		dst_reg->s32_max_value = (s32)dst_reg->u32_max_value;
+	}
+
+	/* mark reg64 unbounded to deduce 64-bit bounds from var_off */
+	__mark_reg64_unbounded(dst_reg);
+}
+
+static void scalar_min_max_mod(struct bpf_reg_state *dst_reg,
+			       struct bpf_reg_state *src_reg)
+{
+	u64 val = src_reg->var_off.value; /* src_reg is const */
+	u64 umax = dst_reg->umax_value;
+	u64 umin = dst_reg->umin_value;
+	u64 umax_rem, umin_rem;
+
+	/* dst_reg is untouched when mod64 zero */
+	if (!val)
+		return;
+
+	div64_u64_rem(umin, val, &umin_rem);
+	div64_u64_rem(umax, val, &umax_rem);
+
+	/* no winding */
+	if (umax - umin < val && umin_rem <= umax_rem) {
+		dst_reg->var_off = tnum_range(umin_rem, umax_rem);
+		dst_reg->umin_value = umin_rem;
+		dst_reg->umax_value = umax_rem;
+	} else {
+		dst_reg->var_off = tnum_range(0, val - 1);
+		dst_reg->umin_value = 0;
+		dst_reg->umax_value = val - 1;
+	}
+
+	/* cross the sign boundary */
+	if ((s64)dst_reg->umin_value > (s64)dst_reg->umax_value) {
+		dst_reg->smin_value = S64_MIN;
+		dst_reg->smax_value = S64_MAX;
+	} else {
+		dst_reg->smin_value = (s64)dst_reg->umin_value;
+		dst_reg->smax_value = (s64)dst_reg->umax_value;
+	}
+
+	/* mark reg32 unbounded to deduce 32-bit bounds from var_off */
+	__mark_reg32_unbounded(dst_reg);
+}
+
 /* WARNING: This function does calculations on 64-bit values, but the actual
  * execution may occur on 32-bit values. Therefore, things like bitshifts
  * need extra checks in the 32-bit case.
@@ -11331,11 +11412,12 @@  static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 	 * and BPF_OR. This is possible because these ops have fairly easy to
 	 * understand and calculate behavior in both 32-bit and 64-bit alu ops.
 	 * See alu32 verifier tests for examples. The second class of
-	 * operations, BPF_LSH, BPF_RSH, and BPF_ARSH, however are not so easy
-	 * with regards to tracking sign/unsigned bounds because the bits may
-	 * cross subreg boundaries in the alu64 case. When this happens we mark
-	 * the reg unbounded in the subreg bound space and use the resulting
-	 * tnum to calculate an approximation of the sign/unsigned bounds.
+	 * operations, BPF_LSH, BPF_RSH, BPF_ARSH and BPF_MOD, however are not
+	 * so easy with regards to tracking sign/unsigned bounds because the
+	 * bits may cross subreg boundaries in the alu64 case. When this happens
+	 * we mark the reg unbounded in the subreg bound space and use the
+	 * resulting tnum to calculate an approximation of the sign/unsigned
+	 * bounds.
 	 */
 	switch (opcode) {
 	case BPF_ADD:
@@ -11407,6 +11489,12 @@  static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
 		else
 			scalar_min_max_arsh(dst_reg, &src_reg);
 		break;
+	case BPF_MOD:
+		if (alu32)
+			scalar32_min_max_mod(dst_reg, &src_reg);
+		else
+			scalar_min_max_mod(dst_reg, &src_reg);
+		break;
 	default:
 		mark_reg_unknown(env, regs, insn->dst_reg);
 		break;