bpf: Restrict bpf_trace_printk()'s %s usage and add %pks, %pus specifier

Usage of plain %s conversion specifier in bpf_trace_printk() suffers from the
very same issue as bpf_probe_read{,str}() helpers, that is, it is broken on
archs with overlapping address ranges.

While the helpers have been addressed through work in 6cff04cfa39c ("bpf: Add
probe_read_{user, kernel} and probe_read_{user, kernel}_str helpers"), we need
an option for bpf_trace_printk() as well to fix it.

Similarly as with the helpers, force users to make an explicit choice by adding
%pks and %pus specifier to bpf_trace_printk() which will then pick the corresponding
strncpy_from_unsafe*() variant to perform the access under KERNEL_DS or USER_DS.
The %pk* (kernel specifier) and %pu* (user specifier) can later also be extended
for other objects aside strings that are probed and printed under tracing, and
reused out of other facilities like bpf_seq_printf() or BTF based type printing.

Existing behavior of %s for current users is still kept working for archs where it
is not broken and therefore gated through CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE.
For archs not having this property we fall-back to pick probing under KERNEL_DS as
a sensible default.

Fixes: fced32942571 ("bpf: add support for %s specifier to bpf_trace_printk()")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Link: https://lore.kernel.org/bpf/20200515101118.6508-4-daniel@iogearbox.net

diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst
index 8ebe46b1af39d88171acc6055759db0b8799ff20..5dfcc4592b23efe335673ea13ba34fbbe4da6338 100644 (file)
--- a/Documentation/core-api/printk-formats.rst
+++ b/Documentation/core-api/printk-formats.rst
@@ -112,6 +112,20 @@ used when printing stack backtraces. The specifier takes into
 consideration the effect of compiler optimisations which may occur
 when tail-calls are used and marked with the noreturn GCC attribute.
 
+Probed Pointers from BPF / tracing
+----------------------------------
+
+::
+
+       %pks    kernel string
+       %pus    user string
+
+The ``k`` and ``u`` specifiers are used for printing prior probed memory from
+either kernel memory (k) or user memory (u). The subsequent ``s`` specifier
+results in printing a string. For direct use in regular vsnprintf() the (k)
+and (u) annotation is ignored, however, when used out of BPF's bpf_trace_printk(),
+for example, it reads the memory it is pointing to without faulting.
+
 Kernel Pointers
 ---------------
 

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index b83bdaa31c7b673e57dbcaada17921846f63cc6d..a010edc37ee02577011f5b385399e9f5376a217f 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -323,17 +323,15 @@ static const struct bpf_func_proto *bpf_get_probe_write_proto(void)
 
 /*
  * Only limited trace_printk() conversion specifiers allowed:
- * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %s
+ * %d %i %u %x %ld %li %lu %lx %lld %lli %llu %llx %p %pks %pus %s
  */
 BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
           u64, arg2, u64, arg3)
 {
+       int i, mod[3] = {}, fmt_cnt = 0;
+       char buf[64], fmt_ptype;
+       void *unsafe_ptr = NULL;
        bool str_seen = false;
-       int mod[3] = {};
-       int fmt_cnt = 0;
-       u64 unsafe_addr;
-       char buf[64];
-       int i;
 
        /*
         * bpf_check()->check_func_arg()->check_stack_boundary()
@@ -359,40 +357,71 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
                if (fmt[i] == 'l') {
                        mod[fmt_cnt]++;
                        i++;
-               } else if (fmt[i] == 'p' || fmt[i] == 's') {
+               } else if (fmt[i] == 'p') {
                        mod[fmt_cnt]++;
+                       if ((fmt[i + 1] == 'k' ||
+                            fmt[i + 1] == 'u') &&
+                           fmt[i + 2] == 's') {
+                               fmt_ptype = fmt[i + 1];
+                               i += 2;
+                               goto fmt_str;
+                       }
+
                        /* disallow any further format extensions */
                        if (fmt[i + 1] != 0 &&
                            !isspace(fmt[i + 1]) &&
                            !ispunct(fmt[i + 1]))
                                return -EINVAL;
-                       fmt_cnt++;
-                       if (fmt[i] == 's') {
-                               if (str_seen)
-                                       /* allow only one '%s' per fmt string */
-                                       return -EINVAL;
-                               str_seen = true;
-
-                               switch (fmt_cnt) {
-                               case 1:
-                                       unsafe_addr = arg1;
-                                       arg1 = (long) buf;
-                                       break;
-                               case 2:
-                                       unsafe_addr = arg2;
-                                       arg2 = (long) buf;
-                                       break;
-                               case 3:
-                                       unsafe_addr = arg3;
-                                       arg3 = (long) buf;
-                                       break;
-                               }
-                               buf[0] = 0;
-                               strncpy_from_unsafe(buf,
-                                                   (void *) (long) unsafe_addr,
+
+                       goto fmt_next;
+               } else if (fmt[i] == 's') {
+                       mod[fmt_cnt]++;
+                       fmt_ptype = fmt[i];
+fmt_str:
+                       if (str_seen)
+                               /* allow only one '%s' per fmt string */
+                               return -EINVAL;
+                       str_seen = true;
+
+                       if (fmt[i + 1] != 0 &&
+                           !isspace(fmt[i + 1]) &&
+                           !ispunct(fmt[i + 1]))
+                               return -EINVAL;
+
+                       switch (fmt_cnt) {
+                       case 0:
+                               unsafe_ptr = (void *)(long)arg1;
+                               arg1 = (long)buf;
+                               break;
+                       case 1:
+                               unsafe_ptr = (void *)(long)arg2;
+                               arg2 = (long)buf;
+                               break;
+                       case 2:
+                               unsafe_ptr = (void *)(long)arg3;
+                               arg3 = (long)buf;
+                               break;
+                       }
+
+                       buf[0] = 0;
+                       switch (fmt_ptype) {
+                       case 's':
+#ifdef CONFIG_ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
+                               strncpy_from_unsafe(buf, unsafe_ptr,
                                                    sizeof(buf));
+                               break;
+#endif
+                       case 'k':
+                               strncpy_from_unsafe_strict(buf, unsafe_ptr,
+                                                          sizeof(buf));
+                               break;
+                       case 'u':
+                               strncpy_from_unsafe_user(buf,
+                                       (__force void __user *)unsafe_ptr,
+                                                        sizeof(buf));
+                               break;
                        }
-                       continue;
+                       goto fmt_next;
                }
 
                if (fmt[i] == 'l') {
@@ -403,6 +432,7 @@ BPF_CALL_5(bpf_trace_printk, char *, fmt, u32, fmt_size, u64, arg1,
                if (fmt[i] != 'i' && fmt[i] != 'd' &&
                    fmt[i] != 'u' && fmt[i] != 'x')
                        return -EINVAL;
+fmt_next:
                fmt_cnt++;
        }
 

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 7c488a1ce318cc52c375b0db7d9db2e353dfa836..532b6606a18a605c34335b81cdb371d0d04a8b45 100644 (file)
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -2168,6 +2168,10 @@ char *fwnode_string(char *buf, char *end, struct fwnode_handle *fwnode,
  *             f full name
  *             P node name, including a possible unit address
  * - 'x' For printing the address. Equivalent to "%lx".
+ * - '[ku]s' For a BPF/tracing related format specifier, e.g. used out of
+ *           bpf_trace_printk() where [ku] prefix specifies either kernel (k)
+ *           or user (u) memory to probe, and:
+ *              s a string, equivalent to "%s" on direct vsnprintf() use
  *
  * ** When making changes please also update:
  *     Documentation/core-api/printk-formats.rst
@@ -2251,6 +2255,14 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
                if (!IS_ERR(ptr))
                        break;
                return err_ptr(buf, end, ptr, spec);
+       case 'u':
+       case 'k':
+               switch (fmt[1]) {
+               case 's':
+                       return string(buf, end, ptr, spec);
+               default:
+                       return error_string(buf, end, "(einval)", spec);
+               }
        }
 
        /* default is to _not_ leak addresses, hash before printing */