From 4800314e19d976d88c87e99bcd37d7635eb57f78 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Mon, 6 Apr 2020 20:11:36 -0700 Subject: [PATCH] samples/hw_breakpoint: drop HW_BREAKPOINT_R when reporting writes Patch series "Unexport kallsyms_lookup_name() and kallsyms_on_each_symbol()". Despite having just a single modular in-tree user that I could spot, kallsyms_lookup_name() is exported to modules and provides a mechanism for out-of-tree modules to access and invoke arbitrary, non-exported kernel symbols when kallsyms is enabled. This patch series fixes up that one user and unexports the symbol along with kallsyms_on_each_symbol(), since that could also be abused in a similar manner. I would like to avoid out-of-tree modules being easily able to call functions that are not exported. kallsyms_lookup_name() makes this trivial to the point that there is very little incentive to rework these modules to either use upstream interfaces correctly or propose functionality which may be otherwise missing upstream. Both of these latter solutions would be pre-requisites to upstreaming these modules, and the current state of things actively discourages that approach. The background here is that we are aiming for Android devices to be able to use a generic binary kernel image closely following upstream, with any vendor extensions coming in as kernel modules. In this case, we (Google) end up maintaining the binary module ABI within the scope of a single LTS kernel. Monitoring and managing the ABI surface is not feasible if it effectively includes all data and functions via kallsyms_lookup_name(). Of course, we could just carry this patch in the Android kernel tree, but we're aiming to carry as little as possible (ideally nothing) and I think it's a sensible change in its own right. I'm surprised you object to it, in all honesty. Now, you could turn around and say "that's not upstream's problem", but it still seems highly undesirable to me to have an upstream bypass for exported symbols that isn't even used by upstream modules. It's ripe for abuse and encourages people to work outside of the upstream tree. The usual rule is that we don't export symbols without a user in the tree and that seems especially relevant in this case. Joe Lawrence said: : FWIW, kallsyms was historically used by the out-of-tree kpatch support : module to resolve external symbols as well as call set_memory_r{w,o}() : API. All of that support code has been merged upstream, so modern kpatch : modules* no longer leverage kallsyms by default. : : That said, there are still some users who still use the deprecated support : module with newer kernels, but that is not officially supported by the : project. This patch (of 3): Given the name of a kernel symbol, the 'data_breakpoint' test claims to "report any write operations on the kernel symbol". However, it creates the breakpoint using both HW_BREAKPOINT_W and HW_BREAKPOINT_R, which menas it also fires for read access. Drop HW_BREAKPOINT_R from the breakpoint attributes. Signed-off-by: Will Deacon Signed-off-by: Andrew Morton Reviewed-by: Christoph Hellwig Reviewed-by: Masami Hiramatsu Reviewed-by: Quentin Perret Cc: K.Prasad Cc: Thomas Gleixner Cc: Greg Kroah-Hartman Cc: Frederic Weisbecker Cc: Alexei Starovoitov Cc: Miroslav Benes Cc: Petr Mladek Cc: Joe Lawrence Link: http://lkml.kernel.org/r/20200221114404.14641-2-will@kernel.org Signed-off-by: Linus Torvalds --- samples/hw_breakpoint/data_breakpoint.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/samples/hw_breakpoint/data_breakpoint.c b/samples/hw_breakpoint/data_breakpoint.c index c58504774118..469b36f93696 100644 --- a/samples/hw_breakpoint/data_breakpoint.c +++ b/samples/hw_breakpoint/data_breakpoint.c @@ -45,7 +45,7 @@ static int __init hw_break_module_init(void) hw_breakpoint_init(&attr); attr.bp_addr = kallsyms_lookup_name(ksym_name); attr.bp_len = HW_BREAKPOINT_LEN_4; - attr.bp_type = HW_BREAKPOINT_W | HW_BREAKPOINT_R; + attr.bp_type = HW_BREAKPOINT_W; sample_hbp = register_wide_hw_breakpoint(&attr, sample_hbp_handler, NULL); if (IS_ERR((void __force *)sample_hbp)) {