From: Vlastimil Babka vbabka@suse.cz
mainline inclusion from mainline-v5.13-rc1 commit a48849e2358ecf1a347a03b33dc27b9b2f25f8fd category: feature bugzilla: 181005 https://gitee.com/openeuler/kernel/issues/I4EUY7
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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We have several modifiers for plain pointers (%p, %px and %pK) and now also the no_hash_pointers boot parameter. The documentation should help to choose which variant to use. Importantly, we should discourage %px in favor of %p (with the new boot parameter when debugging), and stress that %pK should be only used for procfs and similar files, not dmesg buffer. This patch clarifies the documentation in that regard.
Signed-off-by: Vlastimil Babka vbabka@suse.cz Reviewed-by: Matthew Wilcox (Oracle) willy@infradead.org Reviewed-by: Petr Mladek pmladek@suse.com Signed-off-by: Petr Mladek pmladek@suse.com Link: https://lore.kernel.org/r/20210225164639.27212-1-vbabka@suse.cz Signed-off-by: Peng Liu liupeng256@huawei.com Reviewed-by: Kefeng Wang wangkefeng.wang@huawei.com
Signed-off-by: Chen Jun chenjun102@huawei.com --- Documentation/core-api/printk-formats.rst | 26 ++++++++++++++++++++++- lib/vsprintf.c | 7 ++++-- 2 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/Documentation/core-api/printk-formats.rst b/Documentation/core-api/printk-formats.rst index 93c3e48ff30d..dd7ea06eda03 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/lib/vsprintf.c b/lib/vsprintf.c index 931d850891b6..a8d311811d90 100644 --- a/lib/vsprintf.c +++ b/lib/vsprintf.c @@ -2271,7 +2271,9 @@ 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): @@ -2310,7 +2312,8 @@ 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: