diff options
-rw-r--r-- | Documentation/core-api/printk-formats.rst | 26 | ||||
-rw-r--r-- | MAINTAINERS | 8 | ||||
-rw-r--r-- | lib/vsprintf.c | 12 |
3 files changed, 37 insertions, 9 deletions
diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 00d07c7eefd4..9be6de402cb9 100644 --- a/Documentation/core-api/printk-formats.rst +++ b/Documentation/core-api/printk-formats.rst @@ -79,7 +79,19 @@ Pointers printed without a specifier extension (i.e unadorned %p) are hashed to prevent leaking information about the kernel memory layout. This has the added benefit of providing a unique identifier. On 64-bit machines the first 32 bits are zeroed. The kernel will print ``(ptrval)`` until it -gathers enough entropy. If you *really* want the address see %px below. +gathers enough entropy. + +When possible, use specialised modifiers such as %pS or %pB (described below) +to avoid the need of providing an unhashed address that has to be interpreted +post-hoc. If not possible, and the aim of printing the address is to provide +more information for debugging, use %p and boot the kernel with the +``no_hash_pointers`` parameter during debugging, which will print all %p +addresses unmodified. If you *really* always want the unmodified address, see +%px below. + +If (and only if) you are printing addresses as a content of a virtual file in +e.g. procfs or sysfs (using e.g. seq_printf(), not printk()) read by a +userspace process, use the %pK modifier described below instead of %p or %px. Error Pointers -------------- @@ -139,6 +151,11 @@ For printing kernel pointers which should be hidden from unprivileged users. The behaviour of %pK depends on the kptr_restrict sysctl - see Documentation/admin-guide/sysctl/kernel.rst for more details. +This modifier is *only* intended when producing content of a file read by +userspace from e.g. procfs or sysfs, not for dmesg. Please refer to the +section about %p above for discussion about how to manage hashing pointers +in printk(). + Unmodified Addresses -------------------- @@ -153,6 +170,13 @@ equivalent to %lx (or %lu). %px is preferred because it is more uniquely grep'able. If in the future we need to modify the way the kernel handles printing pointers we will be better equipped to find the call sites. +Before using %px, consider if using %p is sufficient together with enabling the +``no_hash_pointers`` kernel parameter during debugging sessions (see the %p +description above). One valid scenario for %px might be printing information +immediately before a panic, which prevents any sensitive information to be +exploited anyway, and with %px there would be no need to reproduce the panic +with no_hash_pointers. + Pointer Differences ------------------- diff --git a/MAINTAINERS b/MAINTAINERS index a9efff1a18c6..e626953f5ed8 100644 --- a/MAINTAINERS +++ b/MAINTAINERS @@ -14319,7 +14319,7 @@ F: kernel/sched/psi.c PRINTK M: Petr Mladek <pmladek@suse.com> -M: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> +M: Sergey Senozhatsky <senozhatsky@chromium.org> R: Steven Rostedt <rostedt@goodmis.org> R: John Ogness <john.ogness@linutronix.de> S: Maintained @@ -19156,7 +19156,7 @@ F: drivers/net/vrf.c VSPRINTF M: Petr Mladek <pmladek@suse.com> M: Steven Rostedt <rostedt@goodmis.org> -M: Sergey Senozhatsky <sergey.senozhatsky@gmail.com> +M: Sergey Senozhatsky <senozhatsky@chromium.org> R: Andy Shevchenko <andriy.shevchenko@linux.intel.com> R: Rasmus Villemoes <linux@rasmusvillemoes.dk> S: Maintained @@ -19807,7 +19807,7 @@ F: drivers/staging/media/zoran/ ZRAM COMPRESSED RAM BLOCK DEVICE DRVIER M: Minchan Kim <minchan@kernel.org> M: Nitin Gupta <ngupta@vflare.org> -R: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> +R: Sergey Senozhatsky <senozhatsky@chromium.org> L: linux-kernel@vger.kernel.org S: Maintained F: Documentation/admin-guide/blockdev/zram.rst @@ -19821,7 +19821,7 @@ F: drivers/tty/serial/zs.* ZSMALLOC COMPRESSED SLAB MEMORY ALLOCATOR M: Minchan Kim <minchan@kernel.org> M: Nitin Gupta <ngupta@vflare.org> -R: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com> +R: Sergey Senozhatsky <senozhatsky@chromium.org> L: linux-mm@kvack.org S: Maintained F: Documentation/vm/zsmalloc.rst diff --git a/lib/vsprintf.c b/lib/vsprintf.c index 92e6085eef15..6c56c62fd9a5 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -2152,6 +2152,9 @@ EXPORT_SYMBOL_GPL(no_hash_pointers); static int __init no_hash_pointers_enable(char *str) { + if (no_hash_pointers) + return 0; + no_hash_pointers = true; pr_warn("**********************************************************\n"); @@ -2242,7 +2245,9 @@ early_param("no_hash_pointers", no_hash_pointers_enable); * Implements a "recursive vsnprintf". * Do not use this feature without some mechanism to verify the * correctness of the format string and va_list arguments. - * - 'K' For a kernel pointer that should be hidden from unprivileged users + * - 'K' For a kernel pointer that should be hidden from unprivileged users. + * Use only for procfs, sysfs and similar files, not printk(); please + * read the documentation (path below) first. * - 'NF' For a netdev_features_t * - 'h[CDN]' For a variable-length buffer, it prints it as a hex string with * a certain separator (' ' by default): @@ -2281,7 +2286,8 @@ early_param("no_hash_pointers", no_hash_pointers_enable); * Without an option prints the full name of the node * f full name * P node name, including a possible unit address - * - 'x' For printing the address. Equivalent to "%lx". + * - 'x' For printing the address unmodified. Equivalent to "%lx". + * Please read the documentation (path below) before using! * - '[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: @@ -3191,8 +3197,6 @@ int bstr_printf(char *buf, size_t size, const char *fmt, const u32 *bin_buf) switch (*fmt) { case 'S': case 's': - case 'F': - case 'f': case 'x': case 'K': case 'e': |